a big THANK YOU all you great hackers.
The iSync Plugin 1.0 took me some time to get it working (it seems to me that N900 Bluetooth is generally a bit brittle) but now it works as advertised.
I have no problem with multiple Mac calenders synced into one single N900 calender, just fine for me.
Also the other limitations (alarm settings) are rather minor.
When I think what problems I had with Mac syncing of other phones in the past (SonyEricsson T610, Motorola RAZR and Motokrazr, Nokia N81) than this all was business as usual... never expect iSync things to work out of the box...