To answer the OPs question:
In the US, a clearance to "maintain 280 kts when able" means that you must remain at 250 kts or below until 10k, then accelerate to 280 kts when above 10k.
The 250 kts below 10k speed limit is implied in all US ATC clearances. ATC cannot issue a waiver for this, and they are expecting you to maintain 250 or less below 10k. Obviously in an emergency you can do what you need to do.
On occasion, you will be issued a clearance such as "leaving 10,000, maintain 250". This simply means they want you to maintain 250 and not accelerate to your normal cruise climb airspeed.
As with all rules, there are exceptions. In this case, if you are more than 12nm off shore (outside US Territorial airspace), ATC can ask you do exceed 250 below 10k.
No need for an FAA attorney...this is the way it is, and has been in my 20+ years of flying.
Nu