There are actually numerous gastropods that have uncoiled shells, most notably perhaps the infaunal, non-mobile Vermetid snails. These use long, whiskery threads to pull organic detritus and small prey into their shells for consumption. They're pretty common on reefs, and many people keeping marine aquaria will have seen their cobweb-like nets.
The problem is that there's no obvious reason why what a Vermetid snail does tells us anything at all about heteromorphs. Yes, both have uncoiled shells, but heteromorphs retain neutral buoyancy throughout their evolution, and there's no sign at all that things like spetae and siphuncle become degenerate, as would be the case had they become non-mobile or even simply animals that crawled about rather than swum or floated.
Cheers, Neale