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If using a Noise-Cancelling-Type Electret-Element is feasible,
You may not need any special High-Pass-Filter-Circuitry,
which may ultimately result in superior Sound-Quality.

Is this possible ?, or
are You restricted to using a commercially-available Lapel-Mic ? ""

A Single-Wire Shielded-Cable is all that is needed to power any Electret-Microphone-Element.

Standard "Phantom-Power", ( if available from your Mixer ),
can be used to Power your Interface-Box,
while still using a very small Single-Wire-Shielded-Cable from the Box to the Mic.

A 9-Volt-Battery, or a Stomp-Box-Board-Power-Supply, will also work just fine.

If You want to use long-Cables,
it's a very good idea to have the Interface-Box attached to the Guitar-Strap.
An Electret-Mic-Element will not perform well pushing a Long-Cable.
For pushing a Signal through Long-Cables
You will need an Amplifier to provide a Low-Impedance-Output in to the Cable.

Do You intend to incorporate "Phantom-Power" functionality into your Box ?,
and will it be the standard ~48-Volts ?,
or do You intend to use a Custom type of Phantom-Power ?

A "2-Box" solution may be the best bet,
one on the Guitar-Strap, and one on the Pedal-Board,
this way You can use any standard generic Guitar-Cable between the 2 Boxes,
and have as much Clean-Gain as You need, with zero loss of Fidelity.
And, the Pedal-Board-Box can be larger to
accommodate 3-Way-Tone-Controls as well as Gain/Volume.

If You expect all of this to work with a commercially-available Microphone
You will need to provide the Operator's-Manual, or PDF with all the Specs.
However, I still highly recommend a
Noise-Canceling-Microphone-Element for your application.

In any case, ANY Microphone used should NOT be rigidly attached to the Guitar-Body.
Some type of Foam Mount needs to be fabricated which will securely hold the
Microphone suspended approximately ~1-inch below the Strings.
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Yes, foam

 

I’ve been searching around for “noise canceling lapel mics”, and can’t find any. Feel free to make suggestions in that regard, sounds interesting!

 

I understand the guitar strap idea, but no guitarist is going to mess around with that. If it’s that important to get a power source closer than a 15’ cable, they’ll just put a 9v in the guitar. Then that is a personal preference, some guys want as little in the guitar as possible, so power from the pedalboard is ideal.

 

A Single-Wire Shielded-Cable is all that is needed to power any Electret-Microphone-Element.

Cheap 2-wires electret mics use the Jfet in the common-source mode so that its output level is a little high then it overloads and produces distortion at about 100dB and louder. I measured 120dB in a Disco that did not sound very loud.

The Microphone Linkwitz Mod (look it up) shows how to re-wire a cheap electret mic using the Jfet as a Source Follower that needs 3 wires and can be put inside a Very Loud drum or piano and maybe in a geetar and not produce overload distortion.
I do not know if there is a mic made like that.

 

I have the PDFs outlining the modifications required of most Electret-Mics
from Texas-Instruments, which allow extremely high SPL levels,
but the ultimate SPL limitation is a mechanical-clearance issue that
causes the Diaphragm to "bottom-out",
and, of course, some Electret-Mic designs make the necessary modifications impossible.
I'll attach the PDFs I have if anybody is interested .........
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Electret-Microphones .........
~99% of them are "Omni-Directional" designs,
this is regardless of whether or not they are in bare-Element-form or
a finished Consumer-Product with a Housing, Cable, and Plug.

The other ~1% are "Noise-Cancelling" designs.
These are normally only available in a "Headphones-with-a-Boom-Mic-attached" form.
This is because they sound very "thin" ( no Bass ) if they are further than
maybe ~6-inches from the Sound-Source,
but they are very good at canceling unwanted Environmental-Noise.

Either style of Mic is available in a Bare-Element-Form,
but Noise-Cancelling designs are not normally available as
a ready to use "Lapel-Mic" with Cord and Plug.
Therefore, to use a Noise-Canceling-Electret-Element would require
the addition of a very small Shielded-Cable, which must be Soldered to the Mic.
The small Shielded-Cable should not be longer than about ~6-feet long before
it is attached to the Pre-Amplifier-Box,
by way of direct-soldering to the Circuit-Board,
or via a 1/8" Plug & Jack connection.

The Pre-Amplifier-Box may be powered by a 9-Volt-Battery,
or, by way of Phantom-Power from a larger Box which also contains Tone and Level-Controls.

The smaller-Box, and the larger-Box, may be connected together by any type
of standard Guitar-Cable with 1/4" Plugs on each end.

The Pre-Amplifier-Circuit-Board does not need a Box if
it is glued, or otherwise attached, to the Interior of the Guitar-Body,
but in this case, a hole must be drilled in the Guitar-Body to
accommodate a 1/4"-Jack for a Standard-Guitar-Cable-Plug.

There is a wide variety of different designs of 1/4" Jacks for Solid-Body-Electric-Guitars
that will work equally well for an acoustic Guitar-Body.
Plastic / Metal / Recessed / Angled / etc., or just a plain 3/8" hole.
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If I wanted to use the Spice simulator in Eagle to simulate the mic, what type of generator would I use for it?

 

A 2-wires electret mic has a Jfet (high impedance current sink) parallel with its biasing resistor which is from 2.2k to 10k.
Then simulate it with a low impedance signal generator in series with a 2.2k to 10k resistor.