Hi Jim,
If I understand you correctly, you want to test whether something like (Beta_rew - Beta_neu) in group A is the same as in group B with 3dttest.
As long as all your regressors are independent (uncorrelated) with each other, theoretically the sampling distribution of these regressors and their linear combinations would be normal distribution. Then it seems to me legitimate to run group analysis of contrast between those LC coefficients with 3dttest.
Gang