just to play devil's advocate...
why not just fuse the HU remote ($1?) and run as many remotes as you want (within reason).
From bcae1 it seems 40mA is about the max turn on current for an amp, and from what I've read, HU turn on's are usually good for a few hundred mA. So even if a given installation happens to use amps requiring 40mA to turn on - it's reasonable to expect 4 or 5 of them wouldn't overload the HU turn on circuit.
If the amps happen to require, say 20 mA - it's quite possible 10 of them wouldn't be a problem for a typical HU.