Would there be no possibility of building a Smallbrook Junction-style interchange station at Worgret Junction itself, with a platform on the Swanage line a short walk from platforms on the main line?
The mainline platforms need only be very basic indeed (no road access, car park or facilities other than a footbridge), the 'Swanage' platform/station could be as elaborate as SR wanted to make it.
I appreciate the land at the Junction isn't flat, the railway is in something of a shallow cutting, but a bulldozer could probably carve out room for platforms without too much difficulty.
That would enable the day-tripper to enjoy the Swanage line, Corfe, and Swanage itself by train all the way, without any of the issues of mixing 'mainline' and 'preserved' services, and no need to involve a bus.
Just a thought anyway!
From a personal point of view, I live in the New Forest and regularly make Saturday trips by train to Poole, Wareham, Weymouth, Lymington and so on. But the thought of getting on a packed bus for the very long and rattling haul from Bournemouth to Swanage (and paying the extra £9.50 for the privilege, with no railcard discount) means I only head to Swanage at most once a year... very disappointed there'll be no direct rail service this year again...