No matter what I try (tried in 4.4 and 4.2 with multiple different rigs). It says nothing is selected when I select a bone. I tricked it to actually assign one bone by selecting it and doning “clear physics from selection”. But as soon as i select something else the option is greyed out again.
Hi @Andrew_Grace,
Welcome to the forum and thanks for reaching out! 
That seems strange…
After pose bones selected, can you then try to run this one-line Python script in Blender Python Console?
from ragdoll.vendor import bpx; print(bpx.selection(type=(bpx.BpxObject, bpx.BpxBone)))
It should print out you current selection, like this:
Funny enough: after running this line once the option became available right away. But not only on the selected bones: on all bones. So problem solved i guess? I still have no idea what was it, though.
Thanks for getting back to us!
Then next time if this happens again, please check the Info panel
And the terminal, or “System Console” if you are on Windows 
To see if there are any error messages.
Also, you may try our beta releases, which contains some bug fixes. The download link can be found in these posts: