It's not a case of legality.
Any employer who is asked by an employee for an unscheduled extra pay increase is free to judge whether they wish to grant the increase or not.
If the employer is aware that the employee can afford to make extra large pension contributions, this could be a factor which would make the employer less inclined to agree to the wage increase.
Tax relief can be applied at every pay period from a prsa in the same way as happens from AVCs.
This is easily set up from revenues myaccount.