Thanks for the ideas!
I think apk919's laser alignment needs a known-good alignment to make targets with. I'm absolutely going to use that method in the future once I have the car set to something I know is right.
Camber I'm comfortable doing myself anyway since on the Elise it's in known increments and has set limits. Toe is a bit harder since the extremes aren't known values, I can't use gravity as a reference, and it's a continuous measure.
Maybe I'll just get it as close as I can with my string rig, leave the diffuser off, and bring it into a "normal" tire shop with a good reputation. I'm just worried about a silly employee scraping the clamshell on the rack or getting excited or twitchy with the clutch and rolling off.