It would be great to have the preview option available on more notices.. currently, it's limited to CHECKIN, CHECKOUT and HOLD_SLIP.
Yes, this would be especially helpful for new libraries coming to Koha who need to test notices before crons are turned on. You can do it by running the message queue in a very targeted way, but that takes more back end work, making it less accessible to libraries. Also, changes to overdue notices need to wait until the overdues run overnight to make sure nothing goes wrong.
For a workaround when testing SendAllEmailsTo could be helpful.