I like the idea of enchanting arrowheads, to make them reusable, however, in doing so, I would preclude them from working properly with enchanted shafts and/or other components. I would also require at least a 24 hour waiting period before a magical arrowhead lends its magic to the whole arrow.
They way that I view it is that every item has an innate matrix/aura, and that when an item is enchanted the magic is woven into that matrix/aura so that it is part of it (bladerunes and such similar temporary magics are slightly different).
The hilt on a sword can be changed out because the hilt itself has absolutely no effect on the blade and its usage. Besides which, the hilt and associated pieces are also much smaller than the magical blade itself.
On the other hand, an arrow is the sum of its parts; head, shaft, and fletching. This is why most magical arrows will have the enchantment on the whole arrow, and not just a piece of it. All three parts are important to the arrow. The matrix/aura for an arrow is a single aura, not three separate auras.
Now, to have an enchanted arrowhead, that can be placed on different shafts, the enchantment is going to have to be modified to either be stronger or to be able to spread and encompass the whole arrow over a period of time (likely 24 hours).
If the enchantment is stronger, that means that the aura will be stronger, and thus it would (IMO) likely overwhelm any enchantments on the shaft.
If the enchantment spreads to the whole arrow, then the arrow will need to be unenchanted for the aura to spread properly.
In both cases, having an enchantment on the head, and one on the shaft will cause problems as the two enchantments both struggle for dominance. Chances are likely that they will short each other out, leaving an enchanted arrow that has no abilities/powers at all.
Before somebody brings up armor and how we did it in Combat Companion, I would like to point out that armor can be worn as a full suit AND that each piece of that suit can be worn/used separately on its own. And that this is why each piece of armor only gets a portion of the overall enchantment on a full suit of armor. (i.e. why each +5 on a full suit only gives a +0.5 to a set of bracers).