While it's nice to have Piccolo Trumpet in A, adding in the Bb version is essential. There is so much trumpet repertoire out there for picc, and plenty of it is played on Bb. This would make composing for trumpet ensembles so much easier as well.
You could start with Piccolo Tpt in A, then in Score Manager change the display name and the Transposition. That's what I do when I encounter a Trumpet in A part that I eventually will change to B-flat
While it is easy enough to make the adjustment, there are enough applications of Bb picc to justify its addition into the library of instruments to choose from. What's one more instrument, especially one that has plenty of usability and can use the same audio files as A picc?
As described by the original poster, it doesn't appear that the Garritan library should be a dependency for this score manager functionality, as Garritan relates more to playback. Playback doesn't need to be VST, and there are other instrument pre-select options that might not map to exactly the same instrument as the pre-select in the Garritan sound library.