That would be tricky, you would have to query the CMS_Class table, find your form, parse the XML of the ClassFormDefinition field to find the default value (or if it's an alternative form, in the CMS_AlternativeForm - FormDefinition). Probably not worth it tbh.
If you really MUST have the default value shared between Admin/MVC, may want to consider a site setting then in the admin using the {% SiteSetting["DefaultUserFieldName"] %} for the default value, and the SettingsKeyInfoProvider for the mvc side.