My understanding for the US, based on the types of regulations that are in force, is that ATC has the authority to allow you to exceed certain speed limits(such as a maximum of 200 knots when within a certain distance of certain airports below a certain altitude) but does not have the legal authority to waive the 250K below 10,000'. Only the administrator can authorize an aircraft to exceed 250 knots(this could be as a publication such as a temporary high speed climb authorization on a SID out of Houston a few years back). One should note that we used to regularly request from ATC a speed of 280 knots below ten thousand feet and ATC would approve it. That doesn't mean that it was legal.