I figured it out, though I feel like there might be a better method.
I filtered the outliner by name, selected all the instances, right click → Edit Components in the Property Matrix → Static Mesh Component
Then for each static mesh, select multiple and hit the reset-to-default for rotation and location.
Selecting every single one at once seems to reset them all to the same default. So for example if I have a StaticMeshN, StaticMeshE, StaticMeshS, StaticMeshW on each instance and select all of them and hit reset, everything will get set to the same value, which may not be default for that one.
For example, if I select ten of each of N/E/S/W and hit reset to default on the N, everything gets the N default value, with the “reset to default” button still showing on the E/S/W components. Instead, I just selected all the N and reset, then selected all the E and reset, then S, then W.