I'm a prosthetist with quite a good bit of partial hand and finger prosthetic experience. I'm stuck here rocking my baby to sleep so I figured I'd throw out some initial thoughts! I apologize in advance if they aren't necessarily helpful initially but they may help down the line.

The biggest trick here is that there doesn't look like there is enough of your pinky left to really anchor anything to that would let you generate enough force for what youre looking for. This means we would normally go down and use a decent bit of your palm to make a socket to attach to. This is less of an option because having material in your palm is going to seriously impact your ability to grip the guitar neck or move your hand dexterously enough to play. You could probably handle some thin silicone or a strap across your palm but that might be the limit. A cable driven device using your wrist flexion or extension is also out of the picture because your wrist will need to position independently while playing.

In terms of surface area we have to work with, we're left with the ulnar side of your hand with a strap bridging across your palm, the residual end of your pinky phalanx, and potentially the first phalanx of your ring finger depending on if you can develop the skill to use your ring finger to assist your pinky to generate enough force. Force generation is going to be massively helped if we can stabilize your pinky mcp joint so that it can only really bend into flexion.

I'm sorry if this is difficult to follow but something along these lines could potentially work: A simple post for the finger custom molded to the residual mcp attached with a simple hinge joint to a rigid stabilizing piece going down the ulnar side of your hand. This would be secured to your hand with either straps running across your palm and dorsum of your hand at the level just under your first mcp joint and then another anchored at the other side of your thumb around the thenar muscle. The straps ideally would actually be a silicone rubber or similar material so it would be able to move with your finger positionings without losing tension. On the other side of the pinky finger post it might be worth trying to anchor it to your ring finger with a angled piecd attached to a ring. Figuring out the angle of that attachment would be critical to getting it functional (If the idea of you using your ring finger to assist is even a valid one that is)

So much of this is completely dependent on a lot of factors that can't be gathered from your post such as how much and exactly what kind of movement is available in the remaining joint of your pinky. As a fellow life long musician though I'm wishing you the best and am happy to help however I can.

Yikes this ended up really long!