Instead of cutting and hemming, I came up with this solution. At first, I didn't use glue or thread, but because it is a work shirt and I need it to stay nice for months of wear, I slowly gave in and attended to details.
I began by making sure the shirt was completely flat and the front and back were even. Then I measured and cut a slit a few inches up the sides. By trial, I found that 6-inches was what I needed, which also happens to be the amount I pull out of the shoulders when I try to get the original hem to sit at the proper place.
To keep the original hem threads from coming undone, I used a glue made for stretchy fabric and dotted it on the ends of the threads. The glue can be seen from the right side, so I probably should have tacked down the threads, but I strongly dislike hand sewing.
Even so, hand sew I did. The slit was stretching wider and higher, so I stitched around the top of the slit. Hopefully, this will keep the fabric from tearing more.
These kinds of t-shirts aren't really my style, but with my alterations, at least they look less dumpy.