After a mythtv-users mailing list discussion, I've added a MYTH_EXTRAOPTS parameter to the mythbackend scripts in the conf.d and init.d directories, to allow service runtime parameters (in the case of that discussion, the --noupnp option). Reproducible: Always Steps to Reproduce:
Created attachment 226949 [details] New file for /etc/init.d/mythbackend as installed by ebuild
Created attachment 226951 [details] New file for /etc/conf.d/mythbackend as installed by ebuild
Sounds ok to me; mythtv herd, please have a look.
Something similar has been added for 0.25.1.