IIRC, most HARP cantrips are nearly identical to 1st level spells in RM. That may not be true in RMU, but it seems so in RM2/RMC. That being the case, you'd have to limit yourself to the ones not duplicated in the RM spell lists, or make 1st level spells in RM your cantrips, or some variation on that theme.
Someone who is more recently well read in RM please correct me if I am wrong.