I have vague recollection of somebody asking on a C4D forum about the ability to have multiple images saved in the Image Viewer -- the most recent render, the previous render, etc. Similar to what LightWave has (one of the vanishingly few areas where LW's interface actually surpasses C4D's...)
In response, somebody mentioned some plugin that implemented a separate image-viewer-like display as an acceptable workaround. As I only want to have some image reference available for modeling (that doesn't get wiped out every time I do a test render) this is probably good enough for me.
But after looking around a bit, I still have no idea what it was named or who made it. Or even if it was shareware or whatware. Anybody care to refresh my memory?