No possible way of playing offline, either when login.minecraft.net is blocked or while wanting to play while traveling to a vacation spot, except with a local hotspot. If implemented, it should only work if you can't login, like in the old launcher, but only for active users/bought accounts.
Linked issues
duplicates 1
Comments 2
Duplicate of MCL-38, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.
Dinnerbone has commented on this repeatedly:
https://twitter.com/Dinnerbone/status/325276146583343106
Offline mode is just under construction in the new launcher. It will be able to play offline later, and the old launcher still works.