Just a bit of follow-up on this for anyone who does anything similar, because I couldn't find much info out there for DIP joint injuries that aren't full-blown avulsions requiring surgery -
I buddy taped the finger for 10 days. I couldn't flex it at all with the splint off - or at least trying to would produce a jolt of pain in the underside of the DIP.
I was worried about the FDP tendon after reading horror stories on the web
so I booked to see a specialist about a week after first injuring it. Saw him, and he booked me an MRI, thinking that it might be an FDP tendon partial rupture. I got the MRI last week and it didn't show anything wrong - the radiologist (who the hand specialist recommended as he often uses him for his clients) said he could see no reason for further symptoms.
The finger's improving and 3.5 weeks after injuring it I can now flex it to the palm without pain, just a lot of stiffness. It's still quite badly swollen from the DIP down to the PIP. No tenderness in the pulleys or collateral ligaments. The DIP joint seems to 'catch' and lock every now and again, then I'll give it a shake and it'll release with a little click. Definitely feels like something's locking up in the joint like a splinter of bone or a lump of something
Did 50 mins of really easy top-roping yesterday without any pain. That made it swell up some more the next day.
I still don't know exactly what I did to it. Basically I think I just hyper-extended and overstressed the DIP, but not enough to fracture it or tear the tendon.