Bryan wrote:

Some RVs work like that - open the faucet at the sink, a little pump
kicks on. Get thee to an RV store.

Good idea, but I'm looking to put together something with close to the
flow of a standard outdoor faucet. I don't think that the RV pumps could
pump 8 feet vertically with any flow rate at all. A 2.5 to 3 gallon
watering can would take forever to fill. I've got an 1/8 hp Little Giant
mag drive pump with enough "oomph" to do the trick, but it's part of my
home brewery, and I won't give it up!

I'm trying to envision some kind of on demand pump system that would
let her fill her watering cans from a rain barrel that is located one
story below her balcony.