Hans,
The option -corStr in 3dLME was added solely for the purpose of capturing the temporal correlation (i.e., AR1) when hemodynamic response curve is the input. So, any other structures for residuals are not supported at the moment.
My experience seems to show that, under most circumstances, fine-tuning fixed- and random-effects are more crucial than the residuals. For example, I might try something along the line-
3dLMEr ... \
-model "DX*VISIT*PHASE*CONGRUENCY+meanFD+(1|VISIT:Subj)+(1|PHASE:Subj)+(1|CONGRUENCY:Subj)+(meanFD|Subj)" \
-qVars "meanFD" \
...
Gang