The reference you are looking for is EU OPS 1, subparts G & H.
There is no specific requirement to ensure that a performance class B aeroplane can meet the requirements of a non standard (i.e. greater than 2.5%) missed approach gradient. This requirement only applies to class A aeroplanes.
Class B aeroplanes are only required to demonstrate an arbitary minimum level of go around performance. This is a steady gradient of 0.75% in the approach climb (going around from 1500ft with one engine inoperative, gear and flaps retracted), and 2.5% in the landing climb (a baulked landing from below DA/MDA with the gear and landing flap extended). For a twin engined class A aircraft, the required gradients are 2.1% in the approach climb and 3.2% in the landing climb. Note that single engine performance is not necessarily guaranteed in either class, even on a standard missed approach.
So using the 5.0% minima is not expressely prohibited, but I would seriously question the wisdom of doing it without checking your aircraft's performance manual first. As 2.5% is the standard missed approach procedure gradient, airfields with a steeper gradient will always publish higher minima for aircraft unable to meet it. If in doubt use the 2.5% minima.