The fact that it's throwing an explicit error means somebody took the time to put it in place for anyone running it on OS's older than Mountain Lion.
There's very likely to be some symbols / API's in the source that are 10.8 only. If you can find those API's and modify those methods to work on older OS's, then you'll have a 10.7 compatible version of Boxen and feel free to contribute your changes back to the community. Be a part of the magic of open source software!