instead of sending a per-vertex tangent you send a per-vertex mirror normal for the "correct" side (ie what is really stored in the normalmap) that would simply be "0,0,0" and for the mirrored vertices it would be the mirror plane's normal. Using that normal being 0 or "somevalue" you can flip the texture OS normal again…
I've never noticed any artifacts or precision issues with object space maps, i get better quality out of them in 100% of cases i've seen, and they compress quite a bit better in our engine too. With tangent space you have all those gradiations to compensate for the lowpoly's normals, and you can get really horrible…
there is no reason it shouldnt work. As said there were animated vertex normals, now with OS-normalmap it's the same, just on pixel level. You can think of those spaces as a "axis cross" with 3 vectors for each major axis. Whether you feed 3 vectors to transform tangentspace to worldspace, or objectspace to worldspace…