lack of documentation and/or teaching material about
the C sources of the kernel.

My experience is 1. that it is very difficult to keep detailed
descriptions of a large program accurate and up-to-date, and
2. that the GAP kernel is very easy to read - the source is
really self-documenting.
So, I don't think the situation is so bad.
I have difficulties imagining who would really profit from such
a workshop.

Andries