The N900 appears to come with a separate layout for the SU-8W in /usr/share/X11/xkb/symbols/nokia_vndr but doesn't use it automatically after enabling the Bluetooth HID. By default it's using the N900 internal keyboard's layout. The Fn key on the SU-8W is not working which suggests it doesn't make the keyboard actually send keypresses of the number keys for example, but the driver has to interpret Fn+Q as 1.

However my keyboard has an AltGr key which the N900 thinks is same as the internal keyboard's Fn key, so AltGr+Q produces 1 and AltGr+Ctrl opens the dialog to enter special characters. Therefore the SU-8W is fully functional although with a very strange layout that probably will be fixed by someone eventually. I'm finding that key repeat works better than with the N810 and no keypresses are dropped, but YMMV.