Would it perhaps be practical to pull the binary files out of SCM and build a small tool that figures out denpendencies and downloads the relevant files from the build server?
Something like this might be exactly what we need, but we don't make money building distributed build artifact caching systems. If we don't make money doing it, we aren't doing it :(
It's often said that developers need to understand the business, and bring forward a business-case, not a technical case.
So: how much money will lose by choosing the wrong SCM today because you're unwilling to change a process you even agree is sub-optimal?
Try to count the number of check-ins, branching, merging etc. the entire team does in a year, multiply by five, and then multiply by even a few seconds of lost time, annoyment and agony pr. event.
I would guess that in that perspective you can afford spending a few days seeing if you can change the build-artifact process to allow you a wider selection of SCMs.
I fully understand this argument, but during the last quarterly all-hands meeting our CEO made it abundantly clear that we aren't doing anything unless it directly brings in more revenue or directly reduces overhead. One of the major reasons for switching from p4 is to reduce overhead, if we have to build a new system for it's replacement to work for us we aren't doing a very good job of reducing overhead.
It sounds like that might ease your constraints.