Okay, I've had a chance to chat with a couple of fresh-water fish owners who have some experience with mollusks--or specifically mussels and this is what they told me.
There is a particular species of "clam" (really a mussel" imported from Asia specifically for the aquarium/freshwater fish trade. While it works well in aquariums and hasn't seemed to pose a threat to fish at all, it is an invasive species that is evidently one of the species of mussels (not the zebra mussel) that has gotten released into the wild and started to choke out the native species.
Another owner I spoke to told me he couldn't say what type of freshwater mussel he had except that (a) he did very well in cold water (below 70 degrees F--so definitely newt temperature), (b) didn't require a lot of moving water; and (c) burrowed down into the substrate (gravel and small rocks). In fact, his "test" for whether the mussels were still alive was to pull them out of the substrate and in an hour or less they'd be buried again in the substrate (ie: if the mussel didn't move he assumed it was dead). He's had one for over a year and several others for at least 3 months with no fatalities. But...he doesn't know the name of freshwater mussel he has (and with over 200+ freshwater mussels in North America, that doesn't narrow it down much).
If I find out any more specific details in terms of a species of mussel that appears to be a good fit for newts (ie: hardy, buries itself, doesn't require an environment very different from a newt's tank), I'll pass that info on.