A few thoughts -
At one time, central bus 21 was extended from Farningham to Brands Hatch for races - there's a photo (looks early to mid 50s) on London Bus Routes (
here)
At least one Maidstone Corporation PD2 got a turn on the Green Line extras to Brands Hatch in the mid 70s when LCBS were having a vehicle crisis, and Dartford's allocation of RMCs was replaced by hired Maidstone buses. It's possible that an Eastbourne Regent V (they were covering at Swanley around the same time) may also have made it - although I've not seen a photo of the latter.
the 'southern area' of LT's country area (and later LCBS) included Grays (GY) garage - they also helped out on the Epsom and Biggin Hill specials.
Picture of Biggin Hill specials (410 and 705 reliefs)
here
It was not unknown before 1970 (and occasionally happened after) for red buses to be loaned to country bus garages at peak summer weekends (or occasionally if there were vehicle shortages or emergencies - think Leatherhead garage got flooded in the late 60s and red buses were brought in to cover for buses that were being dried out) - although I've not seen many pictures of red buses on green line routes (except as emergency replacements for coaches that had broken down in London) - possibly they kept green buses on Green Line reliefs and put the borrowed red buses on local bus routes, so the Green Lines would stand out more in London?)
Ascot races also got quite a lot of relief workings on Green Line 701 until that service faded away.
There were a few special services within the red bus area - Manor House to Tottenham (for Spurs matches), West Ham Stadium (the greyhound stadium, not Upton Park), Wimbledon greyhound stadium - all of which had RMs at some point.
Hadn't been aware of the 160 being part covered by coach operators during overtime bans (also a route very close to my original home) - was it Thames Weald? There's a few pictures on SCT61 of them doing odd temporary routes in the 60s - one
here