I am assuming that the source UD3 member is the same as the OS UD3 member. You can approach this in two ways (that I can think of):
1. Use a BR on a Transformation Rule that executes before the mask to Bypass. You use BRApi.Finance.Metadata.GetMember (as Steven mentioned) to determine if it's a valid UD3 member. If it is set the target to the source otherwise set it to Bypass.
2. Bulk update the Transformation Rules somehow before they are used. This could be a separate BR that you run before month end reporting starts (ie run once) or call it from a TransformationEventHandler (ie runs every time). That BR gets all the base members for the UD3 dimension and updates the Transformation Rules.
My preference would be option 2 as this keeps the audit and drill back. With option 1 you would need a process to identify which UD3 members are being mapped via the BR and add them to the Transformation Rules.
