Is there any way to connect a hose or faucet timer to a humidistat?

Inkbird sells a humidistat for greenhouses that might do what you want.
https://www.amazon.com/dp/B07XGQ7Q53/

For something more complex, a humidistat signal could trip a programmable relay to open a solenoid or motorized ball valve. Any good industrial tech capable guy could make that work for you as well. It's not clear to me what exactly you want to do, but programmable relays can do it, so can an Arduino, PicAxe, etc. There's 101 ways to do what you need.

I think all you need is a solenoid valve. They are available in many different setups (battery, low voltage, line voltage). The easiest to setup would be one that just plugs into an outlet. I found some available by searching Google for "plug in watering solenoid valve" .