[LONGHAIR]

Connecting it to the light switch is a great idea.
I'm thinking that the drain solenoid idea might work in reverse too though? Like it comes on when the compressor is turned off? Drain it at the end of the day, which is far more often than most guys do anyway.
In a commercial/industrial situation the "timer" thing is great. Both they bodyshop where I used to work and the cabinet shop where I work now, employ this method. But, these are very large volume users that are drawing air all day long. The cabinet shop even has two separate screw-type compressors that are tied together with software that alternates which one runs at any given time. At lower levels, it alternates which is the "master" one and the other one sits idle unless the volume exceeds a certain point, then it can kick in and help. Then the next week it will switch to the other as the primary. This also allows for maintenance of the slave unit while it's off-line during working hours. This is an insanely expensive industrial grade set up though.
As a regular guy in a home shop, the timer while running seems like overkill, but I like the idea of the cut-off and "one-time" drain at shut off.
As it is now, I use a manual ball valve on the air lines and drain, but don't really drain it often enough. My usage at home has dropped over the years though.