I looked up teardown guides and videos for the XM5s but didn't find any, so I looked it up for the 4s. I found this iFixit battery replacement guide and it works all the way up to step 9. I took off the covers to see if there was any room for AirTags and (unsurprisingly / very obvious in hindsight), there wasn't.
I put the AirTag inside the left cup (the one that's exposed in the photo), clipped the pad back on, and tried it on my head. The AirTag felt uncomfortable on the lower side of the cup, but I didn't really notice it when it was on the upper side.