I've managed to iron out most of the bugs with my routines now. The only problem now is that I can't find what tile the player is standing on, and as such, can't work out probabilities for how often they should occur.
I was trying to cook up a way for normal grass tiles to have a 1 in 20 chance or something, with "special" grass tiles having a much higher chance. If anyone could point me in the right direction for where to get a tile ID in the RAM/ROM, I will finish this hack off, and get it posted on here for all and sundry to use.
Also, 100th post! :D