Ok, found a duct-tape and a prayer workaround. I found the appropriate installer in the MSI cache (%windir%\Installer) and opened it up for editing using Orca.
In the Condition table, I edited the entry for Pop3Smtp Level 0 from:
((INSTALLSERVER=1) AND (ACTION<>"ADMIN")) OR (INSTALLSERVER=0 AND NOT(MsiNTProductType<2) AND (ACTION<>"ADMIN"))
to:
((INSTALLSERVER=1) AND (ACTION<>"ADMIN")) OR (INSTALLSERVER=0 AND (ACTION<>"ADMIN")
Then I was able to do a modify install and the option showed up. Now I just need to figure out about MsiNTProductTypes and find out how to check what type the machine is currently reporting itself as, what it should be, and how to change that setting permanently. Otherwise, the option will disappear with an unhacked msi.