I was just wondering whether more modern equipment might detect possible differences in phase relationships between localiser signal and 3.6&9 degree g/s signals or some other subtle anomaly
There will be one, not so subtle difference and that is that the 6 degree sideband will reversed sense. Ie, the 90Hz sector is below the 6 degree line and the 150Hz sector above it. This still gives you 0DDM in the middle (a valid centreline) but if you deviate from that (say below) you will get fly down indications instead of fly up indications. All due to the GP beam is formed with antennas and reflections and the like.
As Pottwiddler said though these sideband are failry well supressed these days and the routine flight inspections if ILSs would pick up an anomolies.
The distance/altitude table would let you know though and I guess that you wouldn't "feel" quite right about the approach either. Come to think of it ATC might have also queried what you are doing if they cleared you for the ILS and you are twice as high as you should be!