The Rule Book wouldn't have prevented the evacuation of passengers, either onto a train on the same line or, preferably, alongside on the adjacent line. Remember, though, that the contents of the Rule Book are there to ensure everyone's safety.
Evacuation onto the track is certainly a minefield, but evacuation onto a train on the adjacent line has certainly happened in the past, and might well have been a good idea in yesterday's FGW breakdown.
Back in January this year, a friend of mine was on a Virgin Voyager from Birmingham to Edinburgh, which sat down with the brakes hard on about 3 miles south of Lancaster. They brought in single-line working to get trains around it, but eventually the train was declared a failure. It sounded like the computer onboard said no - as I understand it, they tried coupling another Voyager up, but the fault transferred when coupled, so the only way to move the train was to attach a 57, and for some reason that had to be done without passengers on board (at a guess, the train had to be shut down so that the 57 could release the brakes?).
So, the train in rear (which was a London-Glasgow Pendolino which should have been in front of it, but got delayed and was thus trapped behind it, and was also several hours late by this stage) was run back to the crossovers at Garstang and Catterall, and then brought alongside on the other line, and passengers transferred across - it sounded like they used a disabled ramp, but I'll see if my friend remembers. The Preston-Lancaster line does have the advantage of being absolutely ramrod straight, though; doing it on a curve would be trickier.
There is, however, one key difference with last night's FGW incident: both the Voyager and the Pendolino had automatic sliding doors. If you tried to evacuate an HST to another HST on the adjacent line, would there actually be the clearance to swing the doors open?