Please note that I can fit Modelu GWR and Adlake signal lamps in 2mm scale and it is possible to illuminate them using Fibre Optic cable with certain provisos.
The provisos are:
1) You accept that the amount light will not be as great as with an smd led (which is approx lamp size and shape)
2) It is not possible to completely hide the fibre optic cable as it has to enter through the back light aperture.
3) It is quite tricky to drill out something so small as a 2mm scale signal lamp and consequently there will be an additional cost of £5.00. .i.e. £17.00 per lamp in all.
As and when Alan Buttler (Modelu) does other signal lamps in 2mm scale the same will apply to them.
In 4mm scale you can also specify fibre optic lighting instead of smd leds at the same price. (These have 2 cables per lamp, one for the front and one for the back). However, Fibre Optic illumination is likely to be less bright.
Of course Fibre Optic illumination can be used in the other scales at the same cost as SMD LED.