Hi,
I think it would be great to have a tool to learn Pinyin. My idea is to use the same algorithm/system as for the already existing vocabulary list, but I think I would do it independend from the rest of the vocabulary. Instead of the four parts ("Writing", "Reading", "Tone", "Definition") I propose to use two: Listening and speaking.
LISTENING speaks a syllable. The student has to write down the pinyin (like "Reading") for example hen3. In an more advanced level more than one syllable could be spoken. For example hen2hao3 is spoken, but the student has to get, that hen3hao3 has to be written down (since the tones change). Since it is only about the pinyin, there is no problem with an ambiguous answer - the meaning of the words is of no no interest here.
SPEAKING gives a written pinyin. The student has to say it. Afterwards it is spoken and the student has to decide how good he was. (As in "Definition".)
Since it is not learning words, I would seperate it from the other learning system. Since the amount of pinying-syllables is pretty limited (less then 450) this system could be programmed rather static.
What do you think about it? I imagine it is some work to get it working, but it is not out of the world, is it?