It sounded vague to me and I WROTE the OP! 
You got the jist of what I want to do. This project will always have either a +12 (solar) or +24 power supply available. I'm using one of Murata's 24/12 -> 5V switching regulators on my "motherboard" so that gives me a source of +5 (which I need elsewhere so the Arduino regulator is not going to supply enough current).
I'd like to supply +5 to the Arduino (instead of an unregulated voltage, because +24 is going to make the regulator too hot) and I'd rather not use ANOTHER regulator to plug into the normal power jack on the Mega. If I can just put +5 INTO the Arduino without damaging the (Arduino's) on board regulator, I'm happy.
Is that a little clearer? It still sounds vague but between the two posts, maybe it's clear enough.
Thanks,
len