And since the packages are masked, this can easily result in having mythtv 0.20 but mythweb 0.19 (just happenned to me, I had a typo in packages.unmask), which in turn results in unusable mythweb (invalid protocol version).
And since they are package.masked, either cope with and unmask all required packages or don't use p.masked stuff.
mythweb does not depend on mythtv. You need the same version of mythweb to talk to mythtv. Realistically it should just work, just not provide some features if you have a protocol mismatch. But the coders of MythTV aren't good enough to think of this. I know some people that have mythweb on a diff box then their MythTV box and it works... so it shouldn't be a depend.
I'm sure you don't need my backing, but I also have run mythweb on a system that did not have *anything* mythtv related. So please, no artificial dependancies!