I presume you already have a Zones 1-6 Travelcard.
A Zones 7-9 off peak day Travelcard costs £7.60 with a railcard discount, which is the same price as Zones 1-9. See
http://www.tfl.gov.uk/tickets/14416.aspx LUL ticket offices should be able to sell this, as you say at a BR ticket office putting a station such as Bushey as the origin would seem to be a suitable workround.
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Someone has previously suggested that a Bushey - Zone 9 travelcard might be the best value ticket, but I had some difficulty when trying to book this.
Re-reading, I see the confusion with the Bushey ticket. It should be issued as Bushey to Zones 1-6, route AAA Zones 7-9. AAA stands for 'Also available at'.
If you have an Oyster and are prepared to get off at Moor Park in order to touch in, you could do it for £0.95 each way.
Anyone got the (non Oyster) fare for this? Out and back after 10 a.m. on the same day. Gold travelcard discount (as applicable)