Is there a way when viewing and inventory item to make the “QUANTITY ON ORDER” field hypertext to a list of open PO’s for that item?

Why not just create a sublist for PO's in progress?

I'll have to look into how to do that. My purchasing manager is just hoping to click on that Quantity On Order value to see a list of Open POs for that item.

Well, I have a lot of things I hope for too. haha

That's native functionality you would not want to touch - even if you could. Kevin's path is the proper one, a sublist on the item record for open PO's... there's also the related records tab which you could customize a list for.