I can put anything I need to in the manifest of A, including an import for 'com.sun.el'.
...
Now, the hibernate validator asks for an EL provider, and fails. This is harder to deal with (if you don't know about the externalClassLoader).

That's interesting, because actually this should not use the externally provided class loader. Instead, the HV bundle class loader must see the EL packages, at least that's how it should be. Also that import in your bundle should not be needed. Could share a stacktrace for this? May be a bug on our side.

I didn't find the externalClassLoader. I'm happy to discover that I did a poor job of searching.

It's a relatively new feature which we added for this very use case

I think that, for the Karaf feature, it would be better to omit the problem cases than to call them out with 'wrap', or to make two features. I would be happy to patch that bit.

Yes, a pull request would be welcome. JBoss Logging can be used as is as of 3.3.0; Paranamer I suppose we can just drop - people can add it themselves if needed.

I have a small testbed project on github. I will attempt to make it work using what I've learned here;

Cool; The integration tests in the "osgi" module may be helpful to you, there we test different constellations of loading user-provided resources etc.

assuming success, perhaps I could offer some doc patches?

Yes, absolutely! If you think anything should be clarified (or improved implementation-wise), just send a PR. Thanks a lot!