The FDR would still be operative. Remember, the only thing happening is that the plane "thinks" it's airborne and thus the FDR normally works when airborne.
Indeed if the RAT probe is heated on the ground, it could give high readings to the DFGC. But remember it was disabled, so it should not have been a problem for the subsequent TO.
I strongly beleive that they tried an RTO becuase the landing gear failed to retract. The gear will not retract if the plane arrived in this condition!
If the plane indeed was in airborne mode, remember my words. This is an item in the MD-80's Emergency Abnormal Checklist.
Xander