Ok, this appears to work for me. I can't vouch for it's long term stability, having only created it 20 minutes ago, but it seems to remove this particular bug. Might introduce more, but we'll burn that bridge when we come to it.
Now, the details:
THIS ONLY WORKS FOR THE VERSION OF FORGE THAT COMES WITH THE REC. BUILD OF TECHNIC. 3.3.8.164. You can try it for other forge versions, but it's very likely not to work.
To install it, BACK UP YOUR STUFF FIRST, just in case, then open up modpack.jar with your archive tool of choice and go into net/minecraft/client. Replace the Minecraft.class there with this one:
http://www.mediafire.com/?q1xhqh68o5eyu1p
After that go get some achievements. If you run into any bugs that you think might have been caused by this patch, post them here and I'll see if I can twiddle things.
Now, just a note, this is an ugly hackish patch. The proper way to do this would be to tweak the offending mods so they do their achievement initialization in the proper place. That, however, is far more work than I'm prepared to do on this and contains far more risk of mental instability from people yelling at me than I'm willing to take on. If you want this really fixed, go poke the modders and tell them to make sure their achievement code goes in pre-initialization. Heck, this could be fixed once their 1.3 versions come out, because of the granularity of the new FML hooks, but I haven't checked if they've updated yet.