My apologies for the slow reply; I've been caught up in not-so-fun stuff for a while, but I'm beginning to regain some spare time now.
And, yes, I've thought about the environment variables angle; I've considered adding that to Backend eventually, but I'm not sure yet whether it is actually necessary.
For instance, that capability is technically available in the sense that the command line passed to the system via Backend can call batch files (which is how I typically use it), and of course those batch files can use environment variables as much as they like.
On the other hand, I've got to dive back into the source for Backend anyway to fix a glitch I noticed in how it handles undefined extensions, so I may play with adding the environment vars at that time.
So, the summary answer is that you can sort-of do it already, and I may add a more direct handling of that sort of thing in future.