You can do eccentric pull-ups - that's how most things I've looked at recommend building. Boost or climb to the pull-up position, then let your feet hang and lower yourself as slowly as you can until your elbows are nearly straight - then repeat.

You can also do pull-ups in a semi-supine position to a low bar, sort of TRX-style. It won't work the exact same muscles as doing them vertically, but it will help build.

I need to do some... V, you are awesome as always!