Hmm, interesting idea. it seems to me you want "Lair Undead" in the same way you get Lair Items. I would speak to the DM to combine a ritual and magic item for the purposes of making undead but can only guard a certain place, or have limited movement.
The main thing I've seen with 4E is there is next to no OoC flavour and abilities beyond those useful to dungeon crawling. Having an undead horde under your control is powerful, but then so is having a mercenary squad to look after your keep. If they do not interact with the game flow much there should be a way to do it.