Feats which grant damaging effects that scale based on character level seem to be uncommon, but they definitely exist–for example, for monk-type (Superior Unarmed Strike) and rogue-type (Craven) builds. I can’t find any designed for blaster mage-type builds, however.
Reserve feats effectively scale based on caster level, since their damage is based on your highest available spell level, which isn’t really the same as scaling based on character level. The Shape Soulmeld feat can get you Dissolving Spittle, which you can kinda force to scale off of character level, but only by buying more essentia feats. That’s closer to what I want, but it’s pretty feat-expensive.
So… Is there any other feat that can give heavily-multiclassed character some sort of pseudo-eldritch-blast that actually remains (at least somewhat) useful at higher levels?