Some people say that an electric element for a cookstove doesnt work well for DC. Is this true? Wht could you do
to modify an AC element to make it good for a DC dump current resistor?

 

I know of no reason than you couldn't use a resistive stove heating element on DC.
Who are "some people"?

 

The "stove" may not work on DC, but that is another problem. The restive element should be fine.

 

They may be referring to the switch. A DC switch needs to be much tougher as there is no self extinguishing of the arc as the switch opens.
Often you see the switch has an AC and a DC rating marked on it, and the DC one is quite a bit lower.
And some stoves have induction motors too.