Conor,
Great tool! Thanks for taking the time to work it out. I use something very similar on my osC store with the flaw that one must be registered and logged in to use it. This will be a huge improvement! I've installed it on my test zen store and it appears to function perfectly.
I have just 1 concern. When we take seasonal inventory, our update from sold out can be considerable. The way we're configured in our current store, notice is sent the instant stock goes from 0 to >0 on each item. Therefore, only a few dozen emails are launched at any moment. If I hit the button with hundreds of emails in the queue, it could be flagged as spam at the receiving end. Is there something to modulate the transmission interval of the emails? (I know my newsletter program does that, but I don't know how)

By the way, for anyone wondering if this tool is a good idea, you should know that it's just about at the top of my priority list for migrating to zen. It's huge! It generates a huge spike in sales when we do seasonals and the residual sales continue for days.