I think the following works to combine the estimates.
pROC produces a point estimate for the AUROC as well as a 95% Confidence Interval.
To combine the AUROC from m imputation dataets, it is simply averaging the AUROC.
To create an appropriate standard error estimate and then a 95% C.I., I converted the 95% C.I.s into S.E. Using the standard formulas (Multiple Imputation FAQ, I computed the within, between, and total variance for the estimate. Once I had the standard error, I converted that back to a 95% C.I.
If anyone has any better suggestions, I would very much appreciate it.