RE: Business Alert if Quantity not Available on Transaction Date.
As Business Alerts simply checks your script to determine whether the logic is True or False, sending an alert when True, potentially you should be able to set up a Business Alert as you mentioned, i.e. receive an alert on the PO receiving date if the inventory quantity is not available.
That being said, we don't have any sample scripts that would do this for you.
You may be able to use something like the IV10200 (quantity in) and IV010201 (quantity out) tables or SEE30303 table, then work backwards to a date.
For information on the SEE30303 table, you can use this link:
mbs.microsoft.com/.../MDGP10_HITB
It's possible someone on the Community may have more information on this as well, and can give you some examples.
Thanks