Leander, you _can_ use Pianoteq as an AU or VST plug-in instrument to play back the music. (Thus, no sound-sets are involved!!!)
I haven't actually used Sibelius in a little while now (I'm going for MIDI-only and/or graphic notation these days), but that was my main reason for initially getting Pianoteq! (So I could go all "Sorabji" with it! ;^)
Just open the mixer and select Pianoteq as the playback device. (IMHumO, this is one area where Sibelius could be a bit clearer. Too much crap is buried beneath the surface...) You may have to configure the "playback devices" first. (I've uninstalled the program, so I can't do a quick "open & compare" for you. But the manual explains enough for you to get by.)
I do not believe Harmony Assistant hosts plug-ins (and, quite frankly, it's a bit "behind the times" now), so that would be a dead-end.
"Our developers, who art in Toulouse, hallowed be thy physical-models.
Thy version 4 come, thy new instruments be done, in the computer as it is in the wood!"