Carving out a forward base a long way from home to make it possible to wage war at a long distance seems exactly like the sort of meta-game shenanigan that the strategic capital move rule is aimed at preventing in the first place.
There is an important semantic difference between a forward base and a realm. If your only goal is to create a recruitment base for nobles whom you expect to come back to the parent realm afterwards, then it is a strategic move. However, if you make war in order to take the lands of your enemy and create a new realm there, then a secession is a legitimate way to achieve your goals.
As all rules which deal with intent instead of facts, it's hard to police.
