I did precisely what you are asking about as an experiment a few weeks ago, just to see how it went.
I took cuttings one week into flower, they rooted faster than my regular cuts, maybe because I took larger cuttings than usual. They rooted after a week, I gave them 1 week veg time, which was really a waste since they did not grow in that time, just became more established and they are now on their 5th week of flower.
I have them raised to give me an even canopy and they are coming along ok.
The issues are:- The effect on the donor plant, does it slow flowering? I can't really say but the flowering/bud development on the donor plant does not appear to be any different from another plant of the same strain that is the same length of time in flower.
Reveging time. Does the cutting go back to 24/0 then back to 12/12. Again, I cannot say but I intend to try a batch taken 1 week into flower with no veg and rooting at 18/6 in an attempt to prevent the plants "body clock" being stressed too much. I wouldnt think it will make much difference but it is something I want to try out.
I have not seen your gj Thomas but it can be done and if the plant you have is in a bad way, dont forget, the cutting will be in a bad way too! Having said that, if you feel the yield will be very low, my thoughts are, what do you have to loose by experimenting and giving it a try. All I would say is follow NorCalHal's cloning guide http://www.marijuanapassion.com/forum/showthread.php?t=33009
here, it may save you some heartache.
green mojo and good luck with it. W