DML73, as a custom code for one cart, I'm certain it's possible...

I'd prefer to develop an extra feature that posts the optional products available based on a reference listed in the admin on their respective product pages, say 3 extra products.

Like a cross sell but all of the control remains on the individual product pages.

I suppose then there would need to be some sort of link manager where you could see all the relationships.

And code to handle products out of stock, disabled or deleted from the database.

Is there a coffee pot big enough to keep me going at that point? ;)