Wow, that's quite an odd reaction. Unless the cat does chew on cords, I have to assume it's due to the methimazole.
From a Chinese point of view, it indicates extreme Heat. So a cooling formula might be needed here, meaning WLT might not be helpful for a bit. If the pulse was really superficial, flexible, and easy to feel, I'd wonder about Si Miao San or Hoxsey. If the pulse lifts your finger, I'd think of Qing Ying Tang.
We've had some good success in hyperthyroidism with QYT lately. It cools and moistens and moves, which many hyperthyroid cats seem to need. In addition, its content of Zi Cao appears to be able to allow use of methimazole on a 3 days on, two days off basis, provided QYT is a good fit. Look for overt heat signs, a pounding heart, and a forceful wiry pulse.
Of course, right now, the cat might not want anything. But if you're syringe feeding, try to put some of the herb in there. If the mouth is very painful, a homeopathic may work best at this point. For oral nastiness, you could think of Mercurius solubilis 30C given a couple of times a day. Or if QYT is a good fit, just potentize it and give methimazole a rest. The synergy between the two is more likely to happen when the dose of QYT given is pharmacological, not homeopathic.
Good luck!
Steve