Having tested it pretty thoroughly last night and today, I can report that my fanless scheme won't work as planned for either model.
Without a fan, and even with the top removed, the 33mhz LCIII+ gets very hot in a couple of spots, especially on the 68030; the slower 25mhz LCIII is a bit better, but still too hot for me to be comfortable running it fanless. The heat from the 68030 especially surprised me, as I thought that all 68030s ran cool.
The LC475's 68LC040 gets pretty hot as well, of course. I went to the lengths of spinning down the hard drive via Sleeper, installing a 33mhz 68LC040 from an LC630 board (with the thought that running it at 25mhz would keep it cool), and popping a large heatsink on the processor. I then put on the lid and went away for an hour. After an hour, the whole centre of the logic board was warm, the heatsink was distinctly hot, the PS was getting too warm -- and I figured that this was just not a good idea.
Now, were I do run the LC475 without the top on, and arrange matters so that it got convection ventilation, I suspect that all would be fine. Will ponder that as an alternative possibility. I have been wondering about trying to make myself a sort of 68040-based Cube, so maybe now is the time.