Needless to say, ATC would not permit that.
They might! It's dependent on the civil aviation authority rules of the country. When I was controlling in the UK, I too was surprised at the number of pilots who would request a waiver because it was never granted. However, working in the UAE I am permitted to grant a waiver from wake turbulance separation provided certain criteria are met.
From the GCAA Civil Aviation Regulations part VIII:
1.15 Wake turbulence separation
1.15.1 A specific pilot request for a waiver from any wake turbulence separation may be granted provided –
1. The air traffic controller does not prompt, instigate or invite a
pilot to request a waiver from wake turbulence separation; and
2. When the other aircraft is an ICAO heavy category, or B757
aircraft, the air traffic controller reminds the pilot requesting the
waiver of the category or type of the other aircraft.