My detailed knowledge of locomotive types ran out in the mid '60s with 9Fs and Deltics!
Well, I have just had a happy hour on the tools in my mechanical workshop (aka garage), and I have modified a switch mechanism in the manner proposed above.
The good news is that it works in principle, but could stand a bit more fettling, maybe on another mechanism, to make it work a bit more like the real thing.
Look at the pictures...
The switch mechanism:
View attachment 133282
I left the spring off because it is a tough mean finger busting little thing.
To give you an idea of the scale of this thing, the shaft through the switch is 62mm overall length.
The individual components:
View attachment 133283
And the end view of the modified detents on the shaft:
View attachment 133284
The problem with this is that where I have filed away the detents, in order to get sufficient angle for the little roller which engages with the detents to slide into the HOLD position, I have removed a bit too much metal such that the spring is practically closed up and providing very little force to hold the switch in position.
But hey ho, there has to be several ways to sort that.
JimB