If anyone is interested I solved this challenge by:
1) Copying the VO track only to a separate sequence in the FCP project
2) Exporting that sequence as a full soundtrack
3) Importing that soundtrack file into SoundTrack Pro
4) Editing that single file as a audio file
This worked really well and I was able to perform all sorts of analyses and fixes and compressors etc on the original voice overs. This resulted in a really clean VO track which I then imported back into FCP.