So I did a little experimentation this morning on my recently (seemingly) destroyed h120 and discovered that the problem I was having (ATA -11 error) was, in fact, due to an inadequately-seated CF adapter in the hard drive socket. The player started flawlessly with the original hard drive, so I figured it was something weird with the CF mod. I ground down the little limiters that the manufacturer inexplicably included at the sides of the connector on the adapter -- the ones that were keeping it from fully enveloping the pins of the ATA plug. I slid it into place, and boom, the thing started up without a problem. Yay!
Encouraged by this realization that the problem was unrelated to the RTC mod, I reinstalled said mod and everything still started just fine. Yay!
I haven't installed the custom build that supports the RTC but I have high hopes of it working. Just in case, though, I haven't screwed the case closed yet.
So, finally, I have a few pieces of advice for would-be RTC modders:
1. Unplug the battery before you start soldering. It may seem like a "no duh," but it never actually occurred to me, and was never suggested on the RTC mod page.
2. Consider mounting the RTC chip upside down, just like the directions in the wiki, but rotate it 180 degrees so pin one is on the lower left. This will give more direct wire paths, and fewer crossed wires.
3. Needless to say, be careful in your soldering. That said, it's totally doable if you have the right tools and a steady-ish hand.