While the ability exists for a user to create a custom edit bar, it would be more useful to allow the selection of a custom edit bar per Index. For example, a text can contain characters not found on a standard keyboard of a particular language or require the usage of special templates (such as EB11). It would be ideal to allow the selection of a Custom Edit Bar on an Index that would then replace the standard one in the Pages ns.
An Old English edit bar would contain the additional letters needed for proofreading Old English text and the standard formating ones.
A Latin + Greek editing bar would contain polyphonic Greek and Latin letters with diacritic markers.
To ease, the complexity of implementation, it would probably make sense to allow for the loading of js templates rather than coding in the custom edit bars directly into ProofPage.