To change how keys are mapped I use xmodmap //http://downloads.maemo.org/product/OS2007/xmodmap/. I couldn't find binaries for OS2008 but OS2007 ones work perfectly. Strangely enough the key I use as Alt Gr (to get Polish characters) have different code so I had to edit my xmodmap config file. It was easy to find using xev installed together with xmodmap.
I'm sure that there are instructions how to get it working but if you won't be able to find them, I would be happy to help.

EDIT: I just realised you probably just want to switch between already available layouts and not to create a new one, so xmodmap is not much help I guess.

My Stowaway XTBT01 works better with OS2008 then with OS2007 - It always connects strait away BUT only after I paired it from Bluetooth Keyboard applet. I don't know if it should make any difference but at first I paired it using Bluetooth applet and it only worked the first time. So I deleted my bt keyboard from known devices and paired using BT Keyboard applet. Since then it connects every time. I may be a coincidence though...