This is really sad, that this thread will die in daily forum spam, cause your idea is fantastic. You would need to add few funny pictures to get massive upvotes and bumbs from the community and then maybe Riot would listen.

How much could he hold? Only 3? Would it be like refresh on kill/assist + restore timer?

I really like the idea, but what's with the refresh suggestions? That'd be way too big of a buff for team fights, meaning his base damage and/or ratios would have to be nerfed to compensate.

I love this idea.

Xerath is one of my favorite AP champs, but he gets out his burst and he's done. This would help him out a lot.

pls riot

If Xerath could consistently fire off his ult, as with the ammo system, then it would need a bit of a nerf. Perhaps each consecutive shot could increase the cooldown by a set amount? Kog maw's and Kassadin's mechanic of increasing mana cost also works.
To go along with the whole seige mechanic, perhaps Xerath gains one more ammo when his w is activated?

I support this idea

Yeah, it would simply allow him to do his job - sieging and harassing. At the moment he's nothing more, than burst caster with a bit longer range than usually.