I wonder if someone here could help me.
I would like to have the option of constraining a bone to a specific interpolation regardless of the timeline interpolation status.
I don't know if this could be done with a script or if it needs to be implemented deeper into Moho but it would be a great time saver in my workflow.
Yup, I just do what Victor does. Once you create the key, you can set zero to be the interpolation you want and then delete the key at frame 1. The default for that item will remain whatever you had set frame 0 to.
Also, if this is for a Smart Bone, I make frame one Linear in most cases, so every keyframe I make in the Action will be Linear as I create them (when Use Previous Key is active.) Then, I can set frame zero to be any default interpolation I want for the Mainline. (For a character, I usually set this one to Smooth, but it really can be anything.)