Okay, wasn't expecting this update to be as ugly as it turned out to be. Apparently MCP decided to sweep a bit of ugliness under the carpet, and in the process, broke the way I'd been getting a deobfuscated IC2 jar. Took me a while to figure out how to get it again. On the upside, I fixed some of my workflow in the process, so it's no longer anywhere near as messy for me.
Since we're now on a version that in theory supports block IDs up to 4095, I tried boosting the default ID into that range, but discovered that ModLoader had a heart attack when I tried it. It could be that there's some missing support in Minecraft itself making ModLoader die, but I think it's more likely that ModLoader still assumes 256 block IDs somewhere.
The really good news is that I'm not expecting the 1.2.5 update to be anywhere near as painful, because I already figured out how to handle any more meddling that MCP might do.