alarmd* uses retutime in initfs (sudo gainroot and chroot /mnt/initfs/ retutime) to set the time for the device to wake up. No idea if this would have to keep being set everytime it has woken up but your script could handle that.

* http://repository.maemo.org/pool/dia..._0.5.20.tar.gz (look at rpc-retutime.c)