I haven't tried that specific tactic, but it is interesting that with the way Unholy Blood Ritual is worded, it does affect demons that may have not existed at the time it was cast. For example, I have used it in combination with the Demonhide Mask to give my Mage Melee +1 when attacking, even though he doesn't count as a demon when the ritual is cast and doesn't affect the costs of the ritual. Summoning a Demon after casting the ritual is a little trickier though because they most likely won't get to attack that round without some help, but theoretically it could be a cool trick.
