You could put the EXEC_ONCE command onto the Hijack menu, and run it from there after the network is fully up.
Code:
## In the config.ini file:

[hijack]
;@MENUEXEC RunRsync /bin/rsync ... &>/dev/null

After adding that to config.ini on the empeg, you should then see a "RunRsync" menu entry on the Hijack menu (long press of knob).

Or have the script background itself and poll for networking before starting the rsync.