Huh, just 90 characters; I never noticed that. That should indeed be extended, and will be for the next release.
The good news is that the limit is not checked in the code, it's just the database complaining about receiving too much data. If you can figure out how to widen the datatype of that particular field, you should be all set. It could be as simple as executing the following SQL:
ALTER TABLE jforum_banner ALTER COLUMN banner_name TYPE VARCHAR(1000);
Note that for putting a message on all pages, you can also use the "Public announcement" under Admin Control Panel -> Configurations.