Query MarketPress about current user's purchases

Hi:

I'm trying to do what my business absolutely needs and what I was told "We won't ever be building that into MarketPress." (I guess sometimes we just have to do what we have to do, sorry Aaron - https://premium.wpmudev.org/forums/topic/marketpress-2x-always-adds-shipping-page-despite-no-shipping).

I need to open up a piece of a product post protected by Membership tags [protected] based on the purchase in Marketpress. Very simple idea: someone who HAS A MEMBERSHIP, OR PURCHASED the product CAN SEE what's between [protected] [/protected]. Ok, you guys don't want (for some obscure reason) ever to support selling posts from Marketpress, but this is simple enough and elegant solution.

Now, before I launched heavy artillery, can you at least suggest any nice filters/places to put that in? Again, I'm a logged in user, I open product page /store/product-1, I purchased that product via MarketPress. What's the easiest way to hook this to? For example, can you give me a php/query to find out all purchased product ids, and disable security on 'protected' tag?