MountainProject could really benefit from being open source. It has a strong brand, especially now that it's under REI, so having the code available to others should not pose a business risk.
MP could benefit from distributing the work to the outside world instead of struggling to find employees. There's plenty of overlap between software engineers and rock climbers and people willing to help.
I'd put it on Github, manage the work items to keep the product in line with the internal vision, have your engineers work on what's important to the company, and let outsiders work on whatever they find interesting and important that would otherwise get ignored. At the very least, the bugs will get fixed.