ASK HN: A company I worked for owes me a paycheck, and now refuses to pay.

IMO for purposes of a demand letter on a small matter like this you could almost pick an attorney at random. Call a couple of solo lawyers who have small ads the yellow pages, explain what you want, ask for a quote for the initial demand letter. Also, instruct them to take NO further billable action after sending the letter w/o your express permission (to avoid them racking up another hour's fee to "review the reply" if your former employer sends them a reply). If they get a reply instead of a check, they should simply forward it to you. At THAT point contact your state labor board.

these guys (http://www.gcalaw.com/) are good, affordable and nice people to boot.

Assuming that the other side of this story is somewhat as it has been portrayed, blogging about the issue with the potential to reveal the name of the startup might encourage them to clear up "the misunderstanding".

Heck, you might even just email someone with some equity a link to this story.

Look into California labor laws. Even though old link, I found this: http://www.labor.ca.gov/pdf/lwdalaborlaw99-03.pdf

It states "It provides that salary may be not be used as an excuse for employment termination decisions if it disparately affects older workers."