my 2 cents
I have been watching this time setting option.

I agree that a cron approach is the best way to activate the change.

Write a small script that changes the STORE_STATUS value of the DB and insert into the configuration table.

open_shop cron insert 0= Normal Store
close_shop cron insert 2= Showcase with prices

With this approach it can be out side zen-cart code or write it into the admin as a mod.

Just a suggestion.