Derab, not sure about all airlines but crew are told not to accept any restraint in the cabin which is not approved (ie does not have the decals as mentioned earlier) Though the strap is similar, as it has not been approved for use with airline seatbelts I can imagine you might get knocked back if you manage to get it into the cabin at all (ground staff would probably say no, that is if you mention it- not that I'm suggesting you shouldn't
) But I can pretty much guarantee when the cc do their cabin secure prior to take off it would get noticed pretty quickly...
Not sure why they won't approve the car seats though... have you asked if it is a blanket rule or is it to do with the model you have? I know in my few years of flying I have only ever seen them used twice...
Good point though about trains/coaches etc... maybe you could invent something to be used across the board and make your millions!
As for the aircraft change tables I agree, they're not the best... a good compromise may be to ask the cabin crew if there is an access toilet on board (they are generally bigger and have outward opening doors) if you brought a portable mat and dont mind putting it down on the floor you might be able to do it that way... bearing in mind not the safest and some crew might not allow it but hey... worth a try if you can't fit the kid on the table... those 737 ones are especially tiny!!