I think I figured it out, though using a different method than fslglm. I just plugged the PEs into my spss spreadsheet that already had my nuisance EVs and did a univariate analysis (GLM) with the nuisance EVs as the only parts of the model, saved the standardized residuals, and plotted those against the EV of interest. Things look somewhat better, thought not by much.
I'm more concerned now that an outlier may have lead to some of these seemingly spurious results. One subject's values are consistently way off, compared to the general trend of others (for both of their RS scan results). Removing them from the plot pretty much flattens out the linear trend, regardless of whether I'm looking at the full PEs or the residuals.
In any case, here's my design.png file.