The current cross class healer spells are weak on purpose because DPS aren't meant to be able to heal effectively.
Healers disconnecting and forcing people to wait any longer than 5 minutes doesn't happen often enough for them to need to implement this, and if the healer is bad, having DPS heal isn't going to fix anything. The healer won't learn anything and in fact would be babied even more by having the DPS do the work for them.
Sorry, but I don't think this is a good idea.