The Webhook API worked great for me. And I didn't need to worry about doing anything asynchronously: as soon as user signs up the first time I create the Webhook (I already have their token in session for their normal browsing session). Then Shopify lets me know every time a product quantity changes and I'm good to go!
I do wish the webhooks were triggered a little faster...seems to be a 15-30 second wait right now...