It is quite possible to use a ready-made A switch blade as a “9ft switch”. The blade is actually a scale 12ft long and the rail would need to be trimmed to this length to make the straight switch principle work. The planing length is exactly the same for an “A” and for a “9ft”: a scale 2&3/4in-wide rail comes out at a scale 5ft 6in for 1 in 24 planing. Loose heel fishplates were also a feature of this length of straight switch as they were intended to pivot rather than to flex.
Post-1923 A switches (the GWR didn’t adopt the A switch as a standard, BTW) are “natural” with a 1 in 7 crossing, as the A switch and the turnout curve radius are both a scale 482ft on an “A & 7”. A “9ft & 4” has a radius of a scale 151ft. Selecting a straight switch rather than an A switch takes the scale 482ft of the semi-curved switch heel out of the equation, making the lead of the junction somewhat shorter. However, don’t expect much other than a very small 0-4-0T and a short-wheelbase 4w wagon to get round this sort of junction if they have side buffers: the limiting factor will be end-throw of the vehicles and the prospect of buffer-locking as straight turns to curve and vice-versa.
I’ve just detailed-designed a “9ft & 4” in 12in scale from first principles and it is intended for things like Wickhams and Permaquip trolleys, where the side buffer coupling is out of the picture and it is only to be used by consenting adults rather than Customers.
I don’t yet know what the geometry of the junction leading from the centre shed road to both platforms at Waterloo, Waterloo & City Line, is. Next time you’re standing waiting for a train from Waterloo to Bank, look at how the train, which is fitted with centre couplings, negotiates the junction leading from the centre siding and arrives in the departure platform. The crossing has got to be in the range of 1 in 4 to 1 in 5 and the end throw is quite startling!