LogMiner delegates a lot of work to the database through various stored procedures and triggers. MySQL isn't yet mature enough to support that, so a porting would require to rewrite a lot of code and do more stuff in PHP.
Since I don't really love MySQL, I'm not inclined to spend my spare time working on it, sorry. ;-)
If anyone else wishes to contribute, he's welcome. Otherwise I might consider doing the port as a paid contract job.