Got it! 
It looks like I wasn’t properly cleaning up one of my cell-culling structures, a grid that’s used instead of portal-culling in some places. And said grid held references to level-geometry…
How this slipped the previous memory-leak hunt I’m not sure–perhaps because I was focussing on a different level, one that didn’t use the grid to any great degree.
There’s still a small leak in one level, but I’m guessing that this is related to something specific to that level, and which should thus be fairly easy to track down…