The qmail eclass should support the "doc" use flag. AFAICT all needed is to make the call to qmail_man_install() in qmail_src_install() conditional. Reproducible: Always Steps to Reproduce: This would probably also solve #61116. Maildrop needs to depend on qmail being emerged with "-doc" then to avoid the man page collision.
It wouldn't begin to solve bug #61116 as many other packages are involved. If it's only man pages that collide, then why not simply work around the problem using FEATURES=noman?