
Looks like it was a 1.6 board where the clock capacitor is not know to leak, but all the 3300uF 6V3 electrolytics had started to dome and leak. Replacing them fixed the machine.
I was currently using a 1.4 Xbox, which is know to suffer from a leaking clock cap, also decided to open it up and do some preventative maintenance. Found this instead:

Clock cap was looking & working fine, but all the 3300uF 6V3 electrolytics looked like they were on their last leg. Removed the clock cap (softmod gets the time from the internetz anyway...) and replaced the dying Nichicons.
Every cap on both boards and PSUs looked fine, except for all 3300uF 6V3 Nichicon electrolytics. On the 1.4 there's a 3300uF 10V Nichicon right next door, even that looks fine. Guess Nichicon had a really shit batch 2004/5 of this particular model. And before that their 2V 1F supercaps, I guess.
Probably a good idea to pop open your OG XB and get rid of those. On the 1.6 there's this one cap attached to the mother of all ground planes, total bitch to clear that via even with a desoldering gun. But apart from that it's a real simple job.
While you're in there, absolutely replace the thermal pads on the CPU/GPU as well. Whatever was in there is stone dry crap. The heatsink on the GPU will be 100% stuck and you'll rip the chip of the board before you get it off, but 1-2min with the hot air @ 120C or so and it'll come off.