I fit a lot of worcestre boilers,and they now have a piece of rubber tubing attached to the appliance,that will allow air to be bled from the boiler safely
Its not allways easy to properly vent a boiler correctly,its a little more than bleeding the pump!
To bleed the pump on a combi it does require the case to be removed,the removal of the case will constitute works to an appliance, which is illegal if you are not a registered installer who will hold all the current qualifications .
The fact that the gas is not being touched does not matter
Speaking as one of the above registered installers, if you hold those qualifications you would know the answer to the question you have asked
May be tomorrows sad demsise of CORGI will actually one day stop people messing with boilers and allow for qualified installers to carry out work to a good standard. long live the register