I did call. Southwest told me that if your ticket is a "Wanna Getaway" (cheap) fare, you're not eligible for standby (e.g., getting put on that earlier flight) even if the earlier flight is half empty and your ticketed flight is full.

Of course, that doesn't prove to me that the ticket counter agents wouldn't bend the rules -- as they often to for oversize carry-ons like guitars -- but it means I'd better not plan on that.

I can make the gig without flying early as long as the scheduled flight's not delayed. It'll be a bit tight but doable. There's a risk of missing it, but then I take that kind of risk every time I have a drive-to gig on a Friday afternoon in Seattle (where freeway tie-ups can easily delay you by 45 min, to an hour). Southwest's not usually late on this particular route.

I'll still probably try it. In the past a couple times when I missed my plane on a Wanna Getaway SWA flight, they put me onto a later flight the same day with no extra fee. I'd call that "stand-by" too, but they let me do it. Alaska did the same once when I had one of their cheap tickets.