The oil problem
You can run any engine speed you want as long as you keep the oil pressure up.
Because of the impossibility of getting a roller bearing down a crankshaft, engine bearings are "plain bearings" with polished steel floating above babbitt (a soft metal). The oil pressure makes the bearings "float", so the dynamic forces of pistons being thrown to and fro is transferred by the oil, not hard steel-on-babbitt contact. The babbitt is only there for coldstarts and very occasional unintended bottoming-out. Frequent bottoming-out will destroy the bearings (it sounds a bit like dieseling).
So you must keep oil pressure high enough to float the bearings.
Super slow idling doesn't work on common off-the-shelf engines because they have a simple positive-displacement oil pump, whose flow is coarsely proportional to RPM. This means the lower the RPM, the less oil pressure available for "floating" the bearings. At very low speeds, there's not enough flow to keep up oil pressure, and you can bottom out bearings - especially at heavy throttle.
So, to idle at 100 RPM, you need to solve the oil problem. Large museum engines have an electric auxiliary oil pump, specifically to pre-fill the oil pressure lines (which have leaked back, and are now empty) so you get pressure immediately on engine start. But that is not enough to pressurize - for that, you need a much bigger pump. But it only needs to run at sub-idle.
That pump is going to be a problem. At too low a speed, it becomes an energy net loss, because you're spending more energy running the pump than the engine gives you. Which defeats the purpose of running the engine.
The power pulse problem
Let's say you solve the oil problem. Experience is that engine RPM must be 500-900 RPM for power pulses to average out enough not to annoy customers. You also don't want to hit a resonance frequency of engine, mounts, suspension or road structure (or airbag sensors, ha!) or it will tear them apart.
Others talk about this at length. I'm going to arm-wave it. Because I assume your engine will be direct-bolted to a large electric motor/generator, and you'll be able to use flux-vector control to actively dampen those power pulses: generating during power pulses and motoring the rest of the time, so engine RPM stays constant. You'll need to do that or this won't work.