Finally got around to giving the mythtv ebuilds some testing. You've removed the sed lines from src_unpack and so mythtv fails under multilib-strict on amd64. I know I said I sent patches upstream to handle this, they are now in mythtv svn but did not make it into the 0.19 release. I'll attach the patch here for inclusion in the 0.19 ebuilds.
Created attachment 82092 [details, diff] libdir patch as included in mythtv svn
Created attachment 82094 [details, diff] Ebuild changes Ebuild changes needed to add the patch and the necessary configure argument.
Patch committed at cardoes request.