Why not just use cron for this ?
15 22 * * 6 backup.sh 0
00 23 * * 0,1,2,3,4,5 backup.sh 1
where backup.sh just runs RMAN with the passed parameter, eg
#!/bin/ksh
print "backup incremental level $1 database" | rman target /
Obviously you'd add more logging and appropriate environment setup in your script, but you get the idea