How to make [uc_order] token globally available?
I think you are going about this the wrong way.

You can make the uc_order token globally avaialbe, but it would be a bit messy since some users wont have an order and other people have more than one order.

Instead you should use hook_mail_alter to actually insert the password from the order. You could search for [uc_order:new-password] and if found load the correct order and replace the value with the value from the order.