I have a similar situation, i live in vegas and we do not have crawl spaces or basements here. My tanks are in my living room with no near; water source, drains, or exterior walls. I have been contemplating how to achieve a reliable drip/drain system.
I think the holding tank with sump pumps is the only viable solution. You would need A bulkhead overflow in the sump at running level that would have to drain into a holding tank within the stand. The sump pump would need to run up inside the wall to the attic, across to the nearest exterior wall down the wall and exit preferably to a garden or other something that would benefit from the fish poo. On the other end similarly the water would need to come from the nearest faucet/toilet/etc up the wall, through the attic, back down at the tank and into the sump. Seems simple enough, but the power outtage brings an issue i dont know how to fix...
When power goes out obviously the water fills the holding tank not the sump, no problem there, holding tank just needs to be sufficient size... but when power comes on the sump evacuates all of that water leaving your tank under filled and pumps running dry with nothing more than a drip to fill it. That may leave the pumps sry too long... additionally if power is out too long the drip never stops which leaves the holding tank being huge to accomodate a big what if...
I know this isn't the answer that you're looking for but just some food for thought. I have been struggling with this one quite a bit and hoping to learn from your situation also. In any case good luck