I did it in London in November 2005. Total of less than 90 minutes. Separate line from non-citizen services. No charge. No big deal. I've got a new passport now. And because I collect stamps agressively--dash into Korea for 10 minutes while in transit at Incheon just to get the stamps--now I need more pages. I've never been to the Embassy in Bangkok, so I might give it a try on an upcoming visit. Failing that, Singapore looks like a good option.
Glad to hear that the CBP officer at DFW who was reluctant to stamp me on my return and told me I couldn't get pages added was full of it.