I don't know if this can be done with native log files, but here are a few sugestions.
1: Use a stats script, if this is what it is for. Most convert to HTMl as standard.
2: See the Tcl archive for script that simply convert the text over.
Aditionaly, I am in the mood for writting (mainly because the idea sounds good to me), to make a script that will convert, and do nothing more than place text into classes. It will be up to you to create a style sheet for it (though I may make a default mIRC style look CSS).
How would you want things stored. Line for line, based on the current doc?