I've considered that. Here in Texas it's quite common to have the water heater up in the attic...which sucks for all sorts of reasons. In the 20+ years I've been in our house I've gone through three water heaters. It's easy to tell when it needs replacing because water starts pouring through the ceiling.

When it's time to replace the heater the plumbers charge extra to pull the old one down out of the attic. I've considered leaving it up there and plumbing it into the system like you described...but in my mine I see the result being even more water damage when the tank goes bad.

Hmmm...maybe moving the old tank outside and just connecting it to an outdoor faucet might be a better idea. I just need to make sure it's drained before a freeze but then it won't have water when I'm most likely to need it. Frustrating.
-Blast