> I can seem to find any people "doing" it and I wondered if it was even possible or if there were caveats that I couldn't think of.

Are you interested in the effect of subjective ratings? Usually there are two approaches to accounting for the trial-level association:

1) Model it at the subject level through the "modulation" method (e.g., -stim_times_AM2 in 3dDeconvolve);
2) Capture the trial-level effects at the subject level first (e.g., -stim_times_IM in 3dDeconvolve) and then model the trial-level association at the population level (e.g., 3dLMEr).

The difference between the two is probably small. The first approach is computationally simpler.

Gang