OP here - FWIW:

First, this was a plea for "what do you do when cramps happen?" Not, "why did I cramp?" I described how I stretch because I thought maybe someone had a better technique. I appreciate the suggestions about stretching while riding by changing my position and the angle of my foot, although I already do that (standing, heel as far down as possible, leaning forward as far as possible. By the time I stop, that is having almost zero effect and I am focused on getting off and stretched before I cramp so hard I have a very tough time getting off the bike.

Second, responding to some of the posts:

1. I ride 150-160 miles a week, most of my rides are at least 30 miles, and I've done at least a dozen 50+ mile rides this year. So cramping at 30 miles (not easy but not hard either) is very unusual.
2. I did, however, run into a series of problems that kept me off the bike for 6 days, the longest stretch this year. So this might have merit. Hopefully I won't get many opportunities to do that again!
3. To H with all you "HTFU" people. I am confident I've got more experience toughing it out than most of you in whatever AND I'm asking for help, not idiotic mantras of "you're just a sissy." Who are you to decide I am not willing to suffer enough? I am cramping because of idiocy or chemistry or whatever, but absolutely NOT because I'm not trying hard!
4. I am quite sure I did not ride very differently because of the saddle - I've been searching for a long time for a saddle that doesn't have me stretching and repositioning after 20-30 miles. I adjusted the fore/aft and angle of the seat several times on the ride, searching for the optimal position. I'm an old hand at this - I've done it with a dozen seats. And the best seat so far is a piece of junk semi-comfort seat with a large cutout that I got at a garage sale for $2 (new) - Serfas, no model. I just sold my Brooks 17, I've mostly used the Specialized Avatar, but I have quite a few miles on a similar Fizik (sp?).

Thank all you non-HTFU responders. Thank you very much. I really appreciate your suggestions and observations.
Mustard? I'll have to try that, although hopefully I won't need to.

Ice? That seems worthwhile to try, although it's hard when you're out pounding it out in the boondocks. But if my legs cramp at home (like they did last night), I will try that. The cramps are scary.