No Legal/Regulatory Base Against
This is not a regulatory issue. There is nothing in ICAO SARPS or any EASA GM that requires special approvals. ICAO does not require this to be in the OPS-SPECS as it is not an AOC requirement.
In a risk based approach, the Operator develops a procedure (when to carry out single engine taxi, ramp conditions, weight, etc). They could present the procedure to the NAA for acceptance.
During a regular line check, the NAA then checks to see if what the Operator says they were going to do with it, actually carries out what they said they were going to do. Simple.