Cannot change From when replying

[Originally part of a discussion in the Admin Forum but moved here as looks like a Client specific issue]

Users have full proxy rights to a Workgroup mailbox. When logged in as themselves they can write a NEW mail and change the From field to the Workgroup address. Mail then goes out as if sent from the Workgroup mailbox and has the Workgroup address in the Reply-To field. But when REPLYING to mail that originally came in to the workgroup address it is not possible to change the From field (It doesn't matter whether the incoming is in a shared folder or forwarded to the individual).

I think this was reported in GW2008 as a possible bug, has it still not been resolved or is there a workaround?