I think what JDW is saying is that without a fan you have a Macintosh SE Oven:
The final word of course is, that heat shortens the life of anything. The best way to run any computer is with adequate ventilation with refrigerated air being blow into it. Many Macs I've seen in recording studios back in the day when the monitor cable length was finite, had to be in the same room. But the fan was intolerable noise. So they were encased in iso-boxes which with no ventilation, totally negating the effect of the fan. An insulated A/C conduit was then attached to chill the air and another air-return ran out of it. Those had to be some pretty happy Macs.
Applied to a fanless Mac, if located near an A/C vent, running a feed into the bottom of the Mac would effectively speed convection and expulsion of hot air and probably keep it pretty cool. Then again it would take away from some of the charm of a clean unit sitting on your table. You might be able to feed it though the back of the PDS card knock-out and down inside, though.
Certainly everything you say is correct. What I would add is that my MacPlus without a fan is also extremely warm if left on for a few hours. And don't forget the SE/30 68030 also produces a lot more heat than does a 68000 chip. I would never run a 68030 without a fan. However, given that the 68030 PowerBooks were basically ventless and fanless, most of the heat must be attributable to the power supply & analogue video circuitry in the compact. But, I'm not sure the heat would get trapped exactly. When the air in an enclosed space heats up, it is naturally convective. Meaning the hot air is going to circulate inside the case and exit the most expedient path, no matter how small the opening. Whether this is enough in a striped down SE needs to be tested. A couple of hours running this way shouldn't hurt it and simply check the temperature of the top of the case. It would be nice to compare it to the temperature of the top of a Plus (not over a vent, but the case temperature between the vents). A check between the SE case temp with a fan and without would also be helpful. Knowing the difference between the two SE configs would allow a lot of good speculation about the rated tolerances of the components.
The final word of course is, that heat shortens the life of anything. The best way to run any computer is with adequate ventilation with refrigerated air being blow into it. Many Macs I've seen in recording studios back in the day when the monitor cable length was finite, had to be in the same room. But the fan was intolerable noise. So they were encased in iso-boxes which with no ventilation, totally negating the effect of the fan. An insulated A/C conduit was then attached to chill the air and another air-return ran out of it. Those had to be some pretty happy Macs.
Applied to a fanless Mac, if located near an A/C vent, running a feed into the bottom of the Mac would effectively speed convection and expulsion of hot air and probably keep it pretty cool. Then again it would take away from some of the charm of a clean unit sitting on your table. You might be able to feed it though the back of the PDS card knock-out and down inside, though.