I don't know what you can or cannot do with Sibelius regarding this, but in Finale, you can de-quantize attacks and releases - - putting them exactly where you want them - - while varing tempo in small or large ways. If, like the Vienna Instruments, the virtual instrument you are working with allows it, you can also shape ADSR's dynamically, turn release samples on and off, employ MIDI Volume and Expression, use velocity crossfades, etc. Finale allows you to write and edit almost any kind of MIDI CC data you'd like. You can aslo use the Human Playback feature and write custom preferences for it to suit the sample library you are using.
Unfortunately, due to a contractual agreement with Native Instruments, Finale is currently limited to playing only "powered by NI" instruments directly - - i.e. as plugins within Finale. This, however, does not prevent you from hosting plugins in another program (e.g. Logic, DP etc.) running on the same computer with Finale and accessing them via the IAC drivers - or of course from hsoting plugins on slave computers. (I believe the previosuly mentioned contractual arrangement will come to an end next year.)
Unfortunately, due to a contractual agreement with Native Instruments, Finale is currently limited to playing only "powered by NI" instruments directly - - i.e. as plugins within Finale. This, however, does not prevent you from hosting plugins in another program (e.g. Logic, DP etc.) running on the same computer with Finale and accessing them via the IAC drivers - or of course from hsoting plugins on slave computers. (I believe the previosuly mentioned contractual arrangement will come to an end next year.)