Hi Norbert,
you wil be pleased to know that the HotPot module already works with TextToys. At least, it works with the TexToys html files.
I haven't come across "Marking" exercises before. It will be interesting to investigate.
As you may know, the HotPot module contains some javascript which is inserted into the Hot Potatoes quizzes. The javascript extracts information from the quiz when it is running the student's browser, and returns that information to Moodle. The javascript expects to find the standard data structures used by standard Hot Potatoes quizzes. As long as your modified quizzes have these standard data structures, the results should be returned from them as required.
You may also be interested to know that I soon intend to begin enhancements to extract information from "Michael Rottmeir" quizzes which are based on Hot Potatoes quizzes.
If you would like to send URLs or examples of non-standard Hot Potatoes quizzes you would like to run via Moodle, I would be very interested to see them. My email address is: gordon at-sign kanazawa hyphen gu dot ac dot jp
--Gordon