Editing an Mailbox configuration file
From what I could tell, Mailbox now stays open after it displays the message, and crashes.

Since the crash had a bunch of JSON processes in it, I figured maybe that the lines were encoded, and since Comic Sans MS does not have the same number of letters as Helvetica Neue, I figured that was the problem. I then changed Neue to Bold, and made it the same number of letters.

Since it stil crashed, I believe it is not actually corrupted. I believe there is a encoded hash, or some sort, in the binary. Since the edit changed the hash, that may be the problem.

TL;DR: It is not possible unless you patch the binary in some way.