Within the last days the internal handling (and partially parsing) of config variables has changed greatly. As a side-effect, the following two features are now available: (1) not only environment variables but all config variables can be used via the usual $foo notation and (2) users can freely define variables by prefixing them with $user_. These can be used just everywhere including the expansion, etc. See the manual for more verbose examples. In connection to that, a draft feature has been added which is still subject to change: via some $muttng_ variables, muttng provides static and read-only information to be used in configuration (paths, version information and currently the header caching backend). If there's positive feedback, this will be kept and expanded to have portable config files defined only once.
Also, more usability function has been and/or will be added soon. Muttng already can remind of possibly forgotten files to attach (if promised in the message's text) and will soon strip signatures when preparing replies...