I'm not sure we have a problem.  Sometimes I want to change packages
with MAINTAINER= someone else, and I just email them.  Just declaring
that if the change is arguably reasonable (best guess is that there'd be
no objection) and no answer in 24 hours, or no answer in 72 hours, then
go ahead.

But, it seems easy enough to add a

MAINTAINERSTYLE=strong

as a way to encode the request to contact, or =weak to say "feel free to
modify, or to ask for help, as you please".

(I'd set weak on pretty much all the packages I maintain, so would make
that the default.)