Carrier freezout actually makes this non-workable -- there's a limit to how cold you can make CMOS devices before they stop functioning. To say nothing of the specific heat of liquid helium, which is miniscule compared to LN2
Using a blowtorch as a heat-source is common in extreme overclocking, to heat up VRMs and memory modules up enough to boot the machine, and keep it working.
liquid helium might get into the device itself and cause other problems. (e.g. iphones stop working in the presence of helium)
Due to the Leidenfrost effect, incidental contact of bare skin and LN2 is not usually a problem (it boils off locally and creates a gaseous N2 barrier). Other things superchilled by it will not be as forgiving, however.
You insulate the motherboard. The aim is to allow only die contact the cup your N2 is in.
There are still problems like cpu/board/components not working due to low temperature or condensation. Generally they are solved by better insulation and trying different components until you find ones that work.
Overclocking is 25% cooling and 75% picking parts that can operate at low temperatures and high frequencies.
Don't forget the crazy binning for ram sticks, cpu dies, and graphics cards. There's a reason all the top guys have major sponsorships, it takes a lot of money to find the top 1% of parts.
Of course, the real grandmaster move is to use liquid helium - its boiling point is about 70C colder than nitrogen :)