Call parking ignores orbit parameter

Our Polycom endpoints are configured to use “native” call parking (ie using SIP messages instead of dialplan) and it works well, with one exception. The phone prompts for an orbit, which is passed along to Asterisk in a Refer-To header, but Asterisk ignores it. Instead the call is dropped into the first or next available orbit (based on the value of findslot in features.conf.) I don’t have a SIP trace handy, but it is defined in an expired RFC draft.

Is there anything I can do to get Asterisk to honour the orbit parameter or is it just completely unsupported?