I'm wondering if you can't accomplish some of these points using existing features/modules :

- Provide a "Booking Widget" Search Block
- Create a custom 'product' node when a user chooses to book something from their search results; containing all the relevant info from that search, and add it to the cart.

You could use uc_node_checkout. The node type would be for example "hotel booking" with from date and to date

Allow different Rates & Quantities for every day for each room node
This is the interesting part...maybe module uc_custom_pice could be inspiring (expet that it calculates price only when the product goes to the cart)

"Expire" room availability via cron, Room availability expiration will be configured so that dates equal to and older than the selected expiry date are expired -- expiry date options are Yesterday, Today, and Tomorrow
couldn't stock handle availability ?

Just my two cents

Actually i need something similar but more generic. Have just been looking around for some starting points. it seems that uc_node_checkout and uc_custom_price might be helpfull

Utopically, the best thing would be to have an ubercart hook like hook_calculate_price. I don't know...