That is a good idea. It's an interesting coincidence that you mention this because another add-on that I'm close to releasing is a new 'Blocked Date' attribute type. With that code it would be fairly easy to add this functionality to the Local Pickup add-on. In fact, I think I may eventually go ahead and add that functionality.

Having said that, the 'Blocked Date' attribute is a much more complicated piece of code than the Local Pickup shipping method. (To give you an idea - I spent approximately 4+ times as much time developing the 'Blocked Date'). It supports blocking days of the week, certain days of the year, and handles cut off times (e.g. if the current time is after 1pm, then the current day is blocked).

Thinking about this more... You could do this now without any modifications to the Local Pickup add-on. You just need to add an order attribute of Date/Time time (e.g. give it the name 'Pickup Time'). Assign it to either the 'Billing' or the 'Shipping' set. Then when the user checks out with an order he will be presented with your 'Pickup Time'. The date/time he selects will appear in the order.

Note that there's nothing to prevent the customer from choosing an invalid date/time. That's where my Blocked Dates add-on would come in handy....

I like the idea of the 'Blocked Dates' addon. Will that be an ecommerce-only add-on, or will it work with other Concrete5 functions as well?

Blocked Dates will be a general purpose attribute type. You will be able to use it anywhere you could use a normal date/time attribute (e.g. with users, files, and pages/collections).