Looks like it should work perfect, except...

Well I tried it and there seems to be a glitch of some kind.

here is an example cart link product action w/url:
http://my.domain/cart/add/p39_q1_o5a5_o4a

I enter this into a link (also directly in the address bar) and the system message reports only this:
p39_q1

These were simplified examples, the actual code, as reported by the system message when adding the product manually to the cart is this:
p39_q1_o5a5_o4aEventName_123456

please let me know what you think is happening here