Hi Jeremy,
There's not a great way of doing this to my knowledge. We have a client that has added a 'School purchase orders, click here' link into her shopping cart which then redirects to a page where they download a form and fax it in (see
https://marierippel.infusionsoft.com/cart/oneStepCheckout.jsp? ).
Other than something like that the only other thing I can think of is if you're savvy with the API you might try adding a separate button to your page specifically for PO orders which passes the relevant data to you in a usable format.
Good luck,
Kyle