Conditional generation of files in distrib/sets/lists?

What is the best way to conditionally include files in the MI lists in 
distrib/sets/lists?

For context, I'm preparing to import an exFAT implementation I wrote a 
couple years ago.  I asked permission from board@ because of the patent 
issues, and they approved it subject to conditional compilation of the 
exFAT modules, based on a mk.conf variable.  This works great, but I 
have to manually add or remove the entries in distrib/sets/lists before 
"make release".  Can I perhaps put the files into a package that is 
conditionally included instead?  Is there a better way?

Thanks,

Konrad Schroder
[email protected]