Of course the original LCC, Laker Skytrain across the Atlantic, moved on to offer a First Class shortly before their demise. By all accounts it was not well patronised, and this was on key business routes.
It does seem to be quite incompatible with open seating when boarding the aircraft.
The Easyjet experiment sounds a bit difficult too. Those paying the higher business fares (and Easy/Ryan on-the-day fares are often as high as the likes of BA but get used because they have the right route) are often those arriving at the last minute and prepared to pay more for the right service - but that does not include needing to be at the gate first.