The D50 can use 1.2v NiMH rechargeables -- that's all I ever used. I think you can easily get 10-12 hours off a set, which has always been plenty for me. I can't envision a scenario where I cannot find time to change the batteries given that time window.
That said, I haven't used an external battery pack, and I don't know anyone who has. I do wonder if it will work, though. I got my D50 used and it didn't come with an AC adapter. I tried 4-5 6v adapters I had on hand, including two different ones made by Sony themselves, and none of them worked. It seems like the D50 is very particular on what will work as external adapter.
Please let us know if you find anything that works, but I'd recommend getting a couple sets of NiMH batteries and swapping them out if necessary. I have a couple sets of "pre-charged" NiMHs (aka, enloop, energy-on, etc). These batteries lose very little of their charge over time, so you can keep an extra set or two charged and ready in your bag for months at a time.