Be warned, the less "blatant / open / loud" the greater the chance that things will go wrong if the regent dies - the Land's Choice will intervene, or a Great Captain will inherit part of the realm, or your heir will otherwise fail to properly be recognised - your heir is precisely that, the person that your court, people, etc expect to inherit, fawn over, suck up to, undermine, etc, etc - the spell certainly helps but it isn't perfect.
Also it should be noted that any realm without an heir is inherently seen as weak (one lucky blow will leave it leadership) and has weaker morale (people might not like the particular heir but they do like stability and certainty that comes from the role being filled).
heirship from forcible investiture is particularly prone to failure - simply catching an enemy regent is not the same as winning the war - a long way too it yes, but not the end of the matter.