Hi, that sounds like an excellent idea. One that i would not think off! I live in the West London area.
Sorry for the delay in coming back to this thread - I've been away for a few days.
If you're in the London patch, then I guess travel difficulties will be less than if you lived out in the wilds...
My knowledge of rail replacement around London (can't guarantee that I'm 100% up to date) from the perspective of control staff (rather than bus / coaches and their drivers) is (in roughly clockwise order) -
FGW - use First Rail Support -
http://www.firstgroup.com/firstrailsupport/ - Casual staff, ranging from students to semi retired. Reasonably good pay. Not often blockades as far in as Paddington, though.
Chiltern / X Country - since it's part of DB group, buses tend to be Arriva group in the first instance. Arriva do employ their own control staff, who come under the wing of Arriva Trains Wales. Website
http://www.railbyroad.com/ - I have also seen Arriva The Shires buses (a while ago) seeking casual navigators to accompany drivers at the start of their shifts.
Virgin - rare for a full blockade, but use First Rail Support
London Midland - control staff supplied by Cabline -
http://www.cablinecoachandbus.co.uk/contact.htm - don't think their pay is great.
LOROL, Southern, South Eastern - buses from Go-Ahead London. Control staffing seems to be quite minimal, mainly staffed by Go-Ahead London staff (presume people working rest days) - bus service is usually good, but absence of staff / information is not great. Have known occasions when even station staff haven't known a rail replacement is running.
FCC - also First Rail Support, tend to staff rail replacement operations more comprehensively than most. Not sure what will happen under new franchise.
Greater Anglia - not sure. Have seen Abelio advertise for a rail replacement planning manager, so this may be moving in house.
http://abelliorailreplacement.co.uk/ seems more pitched towards potential bus operators, but may be worth a look.
C2C - haven't the foggiest.
South West Trains - small Stagecoach rail replacement fleet of buses, control staff tend also to be the 'stand by' bus driver rather than someone specific to that task.
All of the above will take on extra staff via agencies for big events. I have also seen adverts on the likes of Gumtree for big DLR / London Underground blockades, but not sure who organises these. And the advertised pay has been pretty basic.
I'm not going to say where / when, but I have had some involvement in this sort of thing.
Bear in mind that the proportion of irate to happy travellers is relatively high, you're dealing with passengers who need more reassurance than the average train passenger because they are making an unfamiliar journey, and like all transport work, it's often odd hours (and I'm not sure quite what the law is about under 18s working odd hours.) In many cases, you'd be working with at least one other person and near to station staff, but you can end up at an unstaffed station, on your own, with only mobile phone contact to a remote control room.
Happy hunting.