Priest Booster.

I have a 70 Priest on my main account.
Would he be a good booster? The only AoE is Holy Nova.

For lower levels, sure, but the higher levels you'll have to do smaller pulls because clothies are pretty squishy against the mid range groups.

at higher levels i suggest not using holy nova unless your boostee's are tanks, then you can simply heal+dps while your boostees tank. of course, you'll have to experiment.. but for instances up to sunken temple even, you should be ok.
also, there's a glyph that will give 50% more damage to your holy nova while healing you for 50% less, so something like that. i'd use that for more dps while not overhealing.