That's a tricky circumstance. Straight away I was going to say no, that's cruel but your circumstance is a bit different than I was expecting reading the title.
God bless you for all that you've done for your animal, easy to see you are a very caring and loving owner. I treated my dog much the same way.
I'm not going to tell you not to do it, that's up to you. My guess is that if he cannot throw up, he will eventually have diarrhea.
The points and effects will be shocking and uncomfortable to him, but if he trusts you enough perhaps the dermorphin will take his pain away.
Though in all honesty, (and I wouldn't really know) I don't think it would help him much long term, but it still might help, and I have no experience with kambo and joing pain.
What I can recommend though is certain herbs like Mormon Tea/ ma-huang (Ephedra) which is used for rheumatism/arthritis.
http://www.herballegacy.com/Brigham_Tea.html
http://herballegacy.com/Joint.htmlHere is a link to another thread I started about healing bone, flesh, cartilage. (Comfrey & Cissus quadrangis are what I use)
http://kambo.me/smf/index.php?topic=611.msg2765#msg2765 Have you tried applying benzocaine or lidocaine gels or sprays to the joints? You can purchase them on amazon and ebay.
Even 25g of pure benzocaine off amazon cheaply and make a ton of your own.
I recommend applying that before administering his burns.
Perhaps add turmeric to his food to bring down inflammation.
Depending on taste and texture some herbs may be better to add to his water or as a tea to the water, and some herb powders/oils may be better to add to the food.
I used to give my dog tons of herbs, oils, teas, spring water, etc and he lived to be quite old.