It seems to me that most meta-ports should not be direct dependencies. Why not generalize this? DEPEND_ALTERNATIVE= USES=python:xy with a specific version
Requiring a change in qa.sh might deter people from using this behaviour in other ports when they probably should.
Well, I thought about going that way, but decided against, I'd rather there be some sort of overview, or central repository, of what ports should not be depended upon than anyone adding this to their port without understanding what it does. Also, I think the list should remain short.
Also, it is somewhat impossible to do it with a variable set in a dependency, because their Makefile is not read when a port is installed, or they could already be installed.
Maybe ports that USES=metaport should get a warning when they are being depended upon, but it's a somewhat different subject.