Oh, anything is possible. The question is whether you want to take it on. SWAG follows:
The ideal, I think, would be some sort of an "Internal/External" mic switch in Settings, and you can of course ask Apple to consider that by using the IPad Feedback Form.
For now, though, you'd have to work on your end of things .if they have a boom mike, it should be a simple matter to figure out how to create an open in that circuit. Amputation would be effective, but ugly.
If it's a little pinhole mike on a controller or knob somewhere, though, it's more of a problem. You'll probably do best to work at the plug end of the cord; strip the insulation back, figure out which wire is for the mike, cut and secure it, re-insulate with heat shrink or some equivalent.
Unfortunately, there may also be a mechanical aspect to this. Perhaps when the socket detects detects the plug being inserted with its extra connector it all happens automatically, in the same way that it does with the internal speakers. In that case you'll also have to change out the plug itself.
This isn't a difficult job but it won't be one that most will either have the equipment for nor the desire to take on. Luckily, don't have to. Any competent electronic repair shop will be able to do this quickly and easily and for not a whole lot of money.
Good luck!
