Apple hardware team looking at Apple software team: You guys, everything OK over there?
I just did the work of the software team for them:
I got Samba 4 working on Apple Time Capsules: https://github.com/jamesyc/TimeCapsuleSMB
If you have a legacy Time Capsule you'd rather not e-waste, you can try this out. Note that this is very much beta quality software, so don't expect it to work on all configurations.