its not impossible as several coach parties won't return extras till the day (for exmaple if people ill), I have picked up tickets to the sold out performances to HOYS before on the day you just have to be lucky with your timing...
I have certainly seen people selling spare tickets outside Olympia (friends ill, large coach trips not sold out) so worth a try. Think I have seen touts as well but would prefer not to buy from them on principle.
When we had unsold Pony Club tickets from our coach trip the box office wouldn't take them back so I doubt they will have returns unless they have changed their policy on that one. So get there early and look out for people selling - worth a go!