May I offer two other points for consideration please?
ICAO Annex 14 and CAP168 both specify safeguarded approach surfaces. The manual for designing instrument approaches also has limiting surfaces. They look a tad like a square funnel narrowing towards the threshold. There is no such internationally agreed equivalent for a wider cone as required by MLS in the format desired. Hence obstacles that would be suppressed from penetrating the APPS to a runway could potentially infringe a crved approach.
Secondly, in the UK at least and at Heathrow in particular, non standard flight paths whether on approach or departure invite negative comment from local consultative committees, so instigating approach paths that flew over previously "dead air" would be a monumental struggle.
Also, how would ATC sequence a BA 319 say coming in left field?
Ah well - seemed like a good idea at the time I guess.
Sir George Cayley