Thank you for your suggestion but : ... I really can't figure how to draw a border around a word that is placed in a sentence? I think there must be a way to do so cause it must be pretty common to draw borders and the fop developpers must have taken that into account.
Please remember that FOP is work in progress and does not have every feature that the XSL:FO spec requires.
It is perfectly possible that this is missing.
If you can check that it works with other XSL:FO processors and doesn;t with FOP then I am sure that the developers would be quite happy to receive a bug report and test case fo file