Use of ILS glidepath signals outside designated area
From a formal aspect.......
ICAO Annex 10 details the limitations on the use of an ILS.
Under ICAO regulations the signal is only assured useable within the appropriate designated area of coverage.
For the glidepath the lateral area is within 8 degrees of the centreline and then only to a range of 10 miles from the threshold. There is a vertical constraint as well.
The glidepath signal is formed in space by bouncing the radiated energy off the ground in front of the antenna and has several anomalies and variables. Signals within the designated coverage areas will have been flight checked.
The radiated signal will, of course, extend much further so beware using it - that will probably not have been examined.
As an aside, the loalizer also has quite strict areas of coverage, the azimuth covrage is only +/- 35 degrees at 17 miles from the localizer. and 25miles within 10degress of C/L.