Existing membership custom field data does not get loaded into On Behalf profile fields

As described here: http://forum.civicrm.org/index.php/topic,22928.0.html

• Create radio button custom field extending Memberships - Any type
• Add offline membership for an organization and set a value for that custom field
• Make current user's contact an employee of that organization with permissioning checked in relationship form
• Modify sample online contrib page id=2 to REQUIRE On Behalf membership signup
• Go to online contribution form (it will be in renewal mode)
• All organization (contact level) fields are prepopulated as expected

However the membership custom field value is NOT prepopulated.