Misplaced Priority:
In cold water and after some time with the reg in her mouth, my wife's jaw will actually lock up. The doc says she has TMJ. Are there any exercises, medications or other precautions she can do to help prevent this? It is very painfull and requires her to end the dive early each time it happens.
The other options are MantaBite mouthpieces, a bit more expensive than the Seacure ones, and also the ones found on Aqualung regs. All three will keep the regulator in her mouth without having to bite so hard, however if she's not consciously clenching her teeth underwater, and if she's the kind that has her TMJ act up because of clenching her teeth, the Seacure probably isn't going to discourage that. It may actually make her clench more, because it would go further back in her mouth, and it's more easier to bite with more force closer to the back without noticing so much effort.
The MantaBite isn't something I've tried personally, so I can't say anything about it you can't find in reviews or on the website - apparently it has normal bite tabs, though. It has a sort of wing that goes between your cheek and your teeth to help hold it in.
http://www.manta-bite.com/
The Aqualung ones, if she's never used one before, use a sort of arch at the top that goes behind your front teeth to hold it in a bit better, and from my experience, are less comfortable to bite hard on than normal bite tabs, but they still stay in better without much force.
How does she equalize? If she does a lot of jaw wiggling, that could maybe also bother her joints. And since you mentioned cold water, how thick is her hood? Maybe it's too cold in there - or too warm.
As everyone's body chemistry is different and I'm not a diving doctor, I can't tell you what muscle relaxants or other meds might help and still be safe to take before a dive, you'd have to ask someone who is.