Ha ha Purple C! Good test. Event is on Sunday 3 May 2015 between 12.00pm and 5.00pm and public transport is never at it's best on a Sunday.
I can't see anything obvious from Ayr however if the visitor was to get to Sanquhar the #246 bus leaves at 12.25pm from High Street and they could get off at Portrack road end then walk for about 15 minutes, arriving at around 1.15pm.
Return on #102 bus (Edinburgh) from Portrack road end at 5.13pm, get off at the Cross at Thornhill (opposite Bank) and then catch the #236 to Sanquhar at 5.55pm arriving there at 6.14pm.
From Dumfries it's a shorter trip - the #246 bus from Whitesands at 10.58am would get them there by around 11.30am (drops off at Portrack road end & same walk). Return on #102 bus at 4.18pm from Portrack road end and back to Dumfries Whitesands by 4.58pm.
So it is possible at least from Sanquhar or Dumfries but needless to say you wouldn't want to miss any buses or it would be a long walk (or wait).
explore-dumfries-galloway.com/transport/…