What's the power loss to a squeaky chain? I've got an almost new chain, but forgot to oil it before today's ride. It would probably have been fine except for repeated dunkings in muddy water early on. Later in the ride it developed a noticeable squeak. Although I carry oil, I didn't particularly want to add oil on top of filth, and couldn't really be bothered (an ill-judged route led to a fair bit of walking/wading so time and cold were both sub-optimal). But in hindsight, could stopping to oil the chain have actually got me home sooner? Was I wasting loads of power? I ask in a general sense, but my setup is Sora 3x9, on a heavy tourer (and not very aero with extra bottles on the forks).

The question of how much power was lost has been answered. I would just add this: if the chain is squeaking, it’s got insufficient lubricant. You are presumably having bare metal rub against bare metal. That will cause a lot of wear, in addition to drivetrain friction.

Even though the chain was dirty, I don’t see how adding lube on top of the chain could have worsened the situation. The lube should penetrate to the rollers and probably stop the metal on metal situation. That said, note the discussion in the comments; you could risk importing more dirt into the rollers, which would increase the abrasion rate. The question about which of the two situations is less bad might only be answerable if we experimented, and we'd probably need a microscope to measure the amount of contamination between the rollers and pins. In either case, the chain won't explode if you let this happen once or twice, and take this as a reminder to keep the chain lubricated.