maybe Moultons are different (and perhaps they are, F’s have enough holes in to start with…) but I don’t like adding holes to bike frames because I have seen so many frames break via fatigue with the cracks running through (or definitely starting from) such holes. In addition they let water into the frame and this causes internal corrosion. I guess if you don’t do many miles and your bike never gets wet these things are less of a concern.
Call me cynical if you like, but I think that the only reason F frames have survived in any numbers is because they mostly don’t see mega miles in all weathers, and are not treated with the complete disdain that most other old bikes see.
If you do put holes into a frame (of any kind) try to avoid the most highly stressed areas, next to joints and/or wherever the bending stresses are highest. Whilst you would most likely be better off without them, IIRC the holes for most of the F-frame factory cable guides were not placed in dreadful positions.
cheers