Solved! (Thanks for all of the suggestions
)
This may not be the best way to do it but...
1) I created a smartlist (To Stack) to specify which m4a files I need to crate stack files for using "[File Type]=[m4a] [Rating]=>=4 [Duplicate]=[Original] [Stack Top]=-1" for the parameters. ("Duplicate" is a custom field that I have(I use it to designate that I've tagged it and it is worth converting; "[Stack Top]=-1" makes the list ignore files which already have stacks)
2) I convert all of the files in the "To Stack" smartlist to 256 bit mp3 leaving the original file and adding the converted files to the library with the files in the same folder as the original.
3) I select all files in my Audio view and select "Autostack Files by Artist, Album, Track# and Name"
NOTE: As files are added and tagged in my library I occasionally go back and repeat steps 1 & 2.
3) I created a smartlist (mp3's for data discs) using "[File Type]=[mp3] [Duplicate]=[Original] [Rating]=>=4 ~d=a" for the parameters. (~d=a forces MC to look at all files since many of the files that I'm after are actually the hidden stack files)
4) I created a smartlist (MP3 CD) using "p=[mp3's for data discs] ([Genre]=[Rock] or [Genre]=[Blues] or [Genre-Sub]=[Blues] or [Genre-Sub]=[Hard]) ~s=680 ~sort=Random ~mix=30%,rating=5 ~limit=-1,7,[Artist]" for the parameters. ("p=[mp3's for data discs]" pulls the files only from the "mp3's for data discs" smartlist; "~s=680" specifies total size not to exceed 680Mb; "~mix=30%,rating=5" specifies that 30% of the files must be rated 5 stars; "~limit=-1,7,[Artist]" specifies no more than 7 tracks per artist)
5) When ready to burn, I simply select all of the files in the "MP3 CD" smartlist and copy to the burn queue
NOTE: I change the parameters on the MP3 CD smartlist to suit my needs. i.e. Genre, Genre-Sub, whatever
If anyone has a simpler way of achieving what I want to do, please let me know.