Does the onmagiceffecthit block type work in scripted abilities?

Yes. It would be onmagiceffecthit SEFF.

What I mean is, could an ability work like this, so dispel would remove the ability, or does the block type only work on objects?

scn MyAbilitybegin gamemode     do stuffendbegin onmagiceffecthit DSPL     player.removespell MyAbilityend

Sorry about that, I see what you're saying now. Yes, that would remove the spell, but only from the Player (which I'm assuming is the desired effect).

Ok thanks. That's what I wanted to know. On the topic of the dispel effect, what would happen if you made a constant effect enchantment or ability with it? Would it only have an effect when first equipped, or would it effectively provide immunity to all spells, friendly and hostile, with magicka costs below the dispel magnitude?