I stumbled on two similar systems that are now commercially available and basically just what is discussed in this thread:
EchoDrive - Bolt-in Hybrid
XL Hybrids - The Simple Hybrid Solution
Both are marketed towards commercial vans. Neither publish prices... but based on the ROI figures, I'm guessing $10,000 for the EchoDrive. There aren't a whole lot of details on either site.
So, while not cheap, it
appears to be a pretty simple system that could install on a wide range of RWD vehicles, possibly even classics.