Thanks for the help! Here is what I put together using the oddly named "Import / Export" (shouldn't it have 'Edit' in that name?):
~mix=1000,20%,{[Genre]=[Classic Rock]},15%,{[Genre]=[Alternative]},20%,{[Genre]=[folk]},10%,{[Genre]=[country]},5%,{[Genre]=[sea shanty]},10%,{[Genre]=[acoustic]},5%,{[Genre]=[bluegrass]},25%,{[Genre]=[celtic folk]},15%,{[Genre]=[celtic rock]},10%,{[Genre]=[flamenco chill]},5%,{[Genre]=[hawaiian]},5%,{[Genre]=[middle eastern]},20%,{[Genre]=[hard rock]} ~sort=[Genre]
Obviously the percents don't add up to 100%, but MC is smart enough to make it work. It is a different list of 1000 everytime, staying true to the ratios. Nice!
It keeps adding the ~sort thing at the end. Wish it didn't do that.
I wanted the wizard to handle this for me, but I think it is a bug. As usual, MC is a bit esoteric, but there almost always is a work around to get what you want done.
I wish the wizard was more robust. And I'm still curious what the 'Limit percentage to' thing does.