enabling maxlength counter for uneditable fields

I need to enable the maxlength counter for the default OG field og_description. Problem is that this field is uneditable from the "Manage fields" page.

Does anyone have a suggestion on how to circumvent the interface and enable maxlength counter for this field? I haven't looked at the module code yet, but basically this could involve changing an attribute value at the theme level or even in the database. My project spec depends it.

Thanks!