I have not been by camel to Tessalit
If you plan to start your trip in Tombouctou, you can probably find a guide who will take you, as long as you agree on the price. Many of the guides know the desert very well, and as long as it is clear to them what you want to do, and you arrange a contract, then it may be possible. I doubt that you will be able to make arrangements to START in Taoudenni though.
I recommend that if you want to find a guide, that you ask for Hamada at the Hotel Bouctou in Tombouctou. (He lives nearby.) Hamada is not a desert guide, but knows of guides who can take you to the desert, and speaks English and French as well as the local languages. Your guide is unlikely to be able to speak English -but may (or may not) know a little French.
The only way to really find out, is to go to Tombouctou and try, but nothing is certain and there is always risk.........