I don't see why the battery status feature wouldn't still work.
Unless Sony deliberately locked-it-off for non-Sony BT headsets.

You wouldn't need a dongle if you used a non-Sony BT headset.
The PS3 already has "standard" (I'd assume) BT integrated.....

It'd be worthwhile looking into at the very least........

To answer your question......
The Sony BT headset should work with N900, so long as it implements standard BT.
Even if it's not 2.1 EDR, the N900 will be backwards compatible.