Are there any plans for any of the Avadon games to have a Linux version ?

A Linux version of Avadon 1 exists, I believe, though I believe you can only get it through a Humble Bundle.

In any case, pretty much every Spiderweb Software game runs well in WINE, so you don't need a Linux version; just install and play the Windows version. (In fact, I believe the Mac versions of Avernum 1-3 sold here are actually the Windows versions wrapped in WINE, as the original Mac versions don't run on modern systems.)

It's the principle of it for me. If they haven't put in the effort and resources to make a native Linux version then I find it difficult to spend my resources on them when others do put in the effort. I understand their a small studio, it's nothing personal.