There are reports that something like this is possible. See link below, post #11 where a traveler didn't want to use the second entry on his double entry tourist visa and so entered on the visa exemption:

http://www.thaivisa.com/forum/topic/...-visa-can-you/

If/when you try it, do not enter the number of the visa on the arrival card and make certain that when you do enter on the visa, it's prior to the visa expiration.

(Can you not obtain a double entry tourist visa in the UK?)

However, if they do "burn" your visa during your 12 hour visit, you can re-enter on a 30 day visa exemption which can be extended by another 30 days at any immigration office for 1900 baht. See second link:

http://www.thaivisa.com/forum/topic/...ion-extension/

Hope this helps.