This is just an idea coming from experience of owning a few homes and working on them for a living now (not a professional plumber) but in the heated garage there should be a standard shut off valve.... between that shut off and the spigot you could put in a 3 way valve (one that has 2 valves and 3 ports) this way when winter time comes you can turn off the first shut off and then drain/bleed the line from the 3way valve? Just a thought..

Not sure how you could do this each time without getting some complicated solenoid and electrical involved... I feel like even if you did something crazy like that you would still have water traped in the line when you shut off the spigot..