Well, a bit of an update...
Using PST Lite, I have somehow managed to enable DUN, where it works again. Unfortunately, I don't know exactly what it was that I changed that fixed it, but it was under the "IS-95/1x_DATA" section for my phone.
And although it's working, it's definitely not perfect. The A930 will only dial up successfully if I force it into CDMA mode. Also, oddly enough, no EVDO services work even when the phone is back in its original state -- that is, in hybrid mode. This isn't a huge deal to me, because it just means that my MMS messages are a little slow in moving, as they are only going over the 1X network.
One other quirk, though, is that the 1X mode is sketchy. Sometimes, to send or receive, say, a picture message, it takes several attempts at sending it before the phone will actually start transmitting the message (either uploading or downloading). It seems to eventually work, though, if I'm persistent enough.
So anyway, that's where things stand now. I backed up my original phone settings, so think I can always revert back to the working 1X/EVDO services but with broken DUN. It's a shame that I can't have EVERYTHING -- that is, using EVDO for messaging (or even reliable 1X, for that matter) and 1X for DUN, but it is nice to know that DUN over the A930 is still possible. I wish I knew exactly what altered setting "threw the switch," for both enabling DUN and crippling the other data, but maybe I'll be able to narrow it down.
If anybody has had any similar results, feel free to chime in!