Tom,
It can be done. It's one of the more advanced techniques of BIAB.
Firstly... before doing anything, save your song with a new name. Once you've done that, use this version to practise with. That way, if things don't work out as well as you'd like, you can always easily get back to the original file.Have a look at the image below.
1. Open the Soloist dialogue window (#1, #2 on the image).
2. Scroll through the window to select your soloist (#3).
3. Select "Custom" (#4).
4. Set the starting bar of your solo (#5) and enter how long the solo should play for (#6).
7. Generate the solo (#7).
If you don't like the solo generated, use CTRL+Z to undo it and then repeat the above to create another solo. It's also possible to delete the solo using the 'Kill' option that's found under "Soloist | Edit soloist track | Form".
Also, the link below will take you to some PG Music video clips that might be useful as you start your journey.
Regards,
Noel