payment options based on category - possible?

I'll preface this question with the fact that I'm not dead set on the best approach to this so I'd appreciate any ideas.
The problem:
My client has two main "types" of products. One is standard pay and ship, the other is a subscription based product which ships new at the first of the month which would require manual offline processing (or would it?).
The solution?:
What I would like (zen) to be able to do is, detect if a product from categoryX is in the cart and if so, only present the customer with the offline processing option.

If this is a possibility or if you have any alternatives I would greatly appreciate it.
If you're a developer who can assist me with such a mod I would be glad to discuss this with you.

What if a customer decides to buy BOTH a conventional product AND a subscription in the same order? ...

What payment method would apply to such an order?

In the event a customer orders both a subscription and a regular product, the entire order would be processed manually.

Can someone please point me in the right direction? Maybe just give me a hint as to which files I should edit or maybe some syntax to detect if a product is of a certain category ID? I'm pushing a deadline and would really appreciate any help I could get. Thanks.

You can control whether or not a Payment module is displayed based on the:
$this->enabled

being true or false ...

So, you can set a condition for that based on what is in the cart ...

One method is to test if any of the Products in the cart use the master_categories_id that is for one set or the other set of Products ...

I believe if you do a search on:
$this->enabled

you should find quite a few modules that give examples of various things that can be done to show or not show modules ...