How to blend joint matrix between two positions relative to bind pose?

[mostly_human]

I have an animation of a character and I need to add animation to deform the skeleton where some joints will translate to a new position over time (think making the arms longer), but I need to move them in relation to the bind pose so that the animation still plays correctly.
Seems like I should be able to use a blendMatrix between the start and end position for each joint but after experimenting I cant figure out where to output this blended transform so that the joints are moved relative to the bind pose. Thank you for any help!

[faline.custodio.da.silva]

@mostly_human 

Thanks for posting!

This sounds similar to FK\IK blend setup. Basically you have two sets of joints for positioning and then a third set to which you mesh is bound to then you could blend between the two positioning setups. This video may be helpful: Tutorial: Creating an FK/IK arm setup in Maya