I did it and it works surprisingly well. My old thermarest developed a delaminated bubble almost in the middle, so became useless.
I cut the healthy half and sealed it, following the instructions in the MYOG article here, and it worked. I gat perfect seal and I'm using it from time to time since then.
The Erin's/Hig's idea to delaminate the place of the cut using iron on inflated pad may save quite a lot of work with removing the foam from the area to be resealed. Give it a try!