Slasher-
Regarding the "legality" -
Here in the US it's NOT legal. FAR 91.129 (e)(2),(3) states that the large/turbine powered a/c must stay at or above the electronic or VASI glideslope between the OM and MM. It makes no difference if it's an instrument approach or a visual, and makes no difference if it's IMC or VMC. There is also no exclusion for this FAR if it's a displaced threshold.
"Normal bracketing maneuvers" are permitted "for the purpose of remaining on the glideslope."
Along with the real world obstacle concerns voiced here, flying low may violate the noise restrictions for Stage III.
Rules may well be different in other states.