Possible to limit buys per category or buys per option?

I am testing a big bunch of ecommerce solutions in order to find one which meets a specific need. Could any friendly soul out there tell me if the following is possible with ZenCart as I despite of big headaches with this have found no event registration or ecommerce solution to this yet:

I have a set of sessions within an event, each session has a limited capacity. Buyers have to be able to register to a the event (has a fee), and to a maximum of three sessions (some with a fee) within the event, IF there is still places left on those individual sessions. I have tried to understand ZenCarts all possibilities but I am still unsure about this: would the above somehow be possible? Is it possible to limit buys from a certain category for instance, to three, is is possible to have "in-stock-limits" or alike for options of a product (the sessios could in that place be options of one product)?

I am VERY thankful for any help on this!

Cheers, Ben

Zen Cart has a folder called extra cart actions that is designed for inserting snippets of code to allowing very high degree of customisation as to how the shopping cart contents are handled. So you could there insert a small script to look at the contents of the cart and if a customer is trying to combine products in a way that is infeasible, take action or generate a warning.