This "request invasion" idea is essentially an in-game gimmick meant to give a way for players to create their own version of William the Conqueror's conquest of England. It should be noted that unlike in the game, William the Conqueror still had to lobby the Pope with his claim to the throne of England, albeit it was a tenuous and shaky one that some argue could best be described as "a steaming pile of bullshit".

The game's intent is to not simply pretend that the William the Conqueror invasion couldn't happen, but instead provide a mechanic that allows players to mimic it with their own conditions. It's incredibly restrictive, however, and you'll find that there are very few times when it is actually reasonable, probable, or useful to actually utilize this particular feature.

Reasonably speaking, given that it was used for and by William the Conqueror, we can safely presume that such a thing did happen at least once. But it should be remembered that this particular method was not a hall mark of Medieval European politics, and was meant by Paradox to let you craft your own story consistently with the game's history.