I picked up the newer revision of the OPOLAR laptop fan (with the temp display) recommended by @hdBerretz (thanks!) for my Omnia and it works like a charm. With the fan at 2/3 speed, it lowers the idle temp by around 11C - air and CPU (with pakon running).
The ambient temp in my flat is around 26C. At idle, the air temp reported by the fan is around 28C; the CPU temp is around 58C.
Regarding acceptable operational temperatures, I realize the CPU can handle rather high temperatures without issue, but I’m concerned for the mSATA SSD, which sits right next to the CPU, and is more sensitive to heat.
I have no idea what the temps are like inside the case under load, but I feel like a CPU running at 100C for any length of time could make the SSD dangerously hot, particularly if the SSD is also under load.