I'm not familiar with FreeBSD, and much less with its emulation layer...
If they are X11 OpenGL games, I see no reason why the viewer won't run at least as good as they do.
Don't be fooled: the Cool VL Viewer runs modern viewer code. The UI was simply kept the same as in v1 days (but even the UI code got modernized), I fought bloat to keep it fast, and I optimized and cleaned up the viewer code too. But the said code is in no way "ancient" (it's in fact much more recent than other TPVs' or even LL's official viewer; for example, it uses boost fibers instead of boost coroutines, which are only in use in LL's viewer-vs2017 code for now (i.e. a project viewer), and even then, I'm using boost v1.70 when LL uses v1.69...).
No, the advantages of the Cool VL Viewer for a port to another OS reside in smaller (and cleaner) code (half the sources size compared to Firestorm) and a stand-alone build system...