Oh, you have already begun the work. Good, then you can continue improving
it.
No, there needn't be any rules checking. The PGN parser could accept
anything, e.g. 5.Xg1-c7. In this way all variants are supported. It would
be great to have such a tool. Then people could begin to analyse chess
variants, using text comments. In the PGN header one must introduce a
board-type tag (e.g. BOARDTYPE 5). See the PGN standard here:
http://www.tim-mann.org/Standard
The chess variant PGN standard would accept M, X, Z, R, etc., as piece
denominators. One needn't have special gifs for every piece. One could
have generic gifs, too.
The program should be open license so that people also can use it for
commercial purposes (e.g. create chess variant CD:s).
/Mats