When we carved out the first meta API (cart_item)  the model we created included the hooks that fire whenever there is a update or get request for a meta item.  It is the meta item key prefixed with 'wpsc_got_customer_meta_'.

As another example let's say we wanted to eliminate the need for a cart item meta that has the key 'sku' we would create the two functions that fire on 'wpsc_got_cart_item_sku' and 'wpsc_update_cart_item_sku'.