Why not just call it "Protection from Unholy," define that as undead and extraplanar creatures (plus whatever other monsters you think the spell should protect from) and leave it at that?
This is a good question, especially when one looks at Protection from Evil in Holmes and Dispel Evil in Cook:
Protection from Evil — This spell hedges the conjurer round with a magic circle to keep out attacks from enchanted monsters such as elementals, invisible stalkers, demons, etc.
Dispel Evil — This spell will banish or destroy any enchanted or undead monster that comes in range if the creature fails its saving throw vs. Spells.
Evil is clearly equated with enchanted monsters and the undead. Therefore, Roger's question deserves an answer.
It's in the game. This is particularly important given this thought experiment. I am not trying to create my own definitive version of D&D with all the house rules that I think are best for my game Rather, I am trying to create the one that I would have played back in 1981 with only the Holmes & Cook editions available to me, and given a slight prejudice for Holmes over Cook when there is a difference. As such, I need to figure out what exactly these rule-sets mean by alignment (especially since it does have mechanical function).
It has a wargaming pedigree. Alignment has its roots in Chainmail. This is the origin of its mechanical properties and consequences. As I've pointed out before, I am as much of a war gamer as I am a role-player (in some ways more so). In fact, there are plenty of scenarios where I would choose an evening of war gaming over an evening of role-playing (especially if minis were involved). As such, I have a soft spot for those elements of D&D that harken back to its wargaming roots.
Unholy is an alignment. The reason that the Cross has the ability to ward off the undead is not because of the Cross, but rather the God who willingly was crucified upon the Cross in order to defeat death. In other words, the Cross is able to drive off the undead because God Himself works through the Cross. There is a catch, however. If we do not believe in the power of the Living God to affect our lives, our own pride and sin get in the way. Only by properly aligning oneself with God do we allow God to work in and through us.
Having spent much time with the saints by reading their lives, as well as spoken with priests who have had to do exorcisms, I know that evil is a real thing. It can be seen, felt, heard etc. (thus, being able to "detect" it isn't necessarily meta-gaming, especially if you skin it with a "I have a bad feeling about this" kind of vibe). It has power, but only if we allow it to — by aligning ourselves with it. Therefore, as much as I'd like to dump the whole alignment system because it doesn't work as nicely as I'd want, there is a theological reality to it that I, as a Christian, truly appreciate.
This is why I keep revisiting the idea. Despite my reservations, I think it is useful and potentially evocative enough to keep around.