Hi Bob,
XSL-FO uses the TeX-based formatting engine, as FOSI does. So I'm inclined to think Touchup PIs are handled the same with XSL-FO as with FOSI, although I cannot find any documentation on that for XSL-FO. In case it may help, I have attached PDF with everything I know about Touchup PIs and FOSI.
Good luck!
Suzanne