unfortunately, its never going to happen, Swanage excepted, possibly a few other similar restricted arrangements in the coming decades.
The crashworthiness of some 1st gen isn't as good as Mk1 coaches, DMUs have a lot of doors (secondary locking arrangements?), opening windows (leaning out opportunities?), AWS/TPWS required, along with OTMR and data recording, remember they are all max speed 70mph and vacuum braked so what is the contingency plan if it fails with limited pathing, limited train length, minimal catering opportunities so limited income, most charters make money from the dining elements, which makes the charter worth running.
Don't get me wrong, would love to see A 127 depart St Pancras, a 115 at Marylebone and even a 114 doing Sheffield - Skegness, but can't see it ever happening. however, if anyone with plenty of £ does come along, I can help with the previously mention units availability...