Dave S wrote:
3) declare an emergency in which case you can disregard just about
everything but the laws of physics.

Not quite. The rule says:

91.3 Responsibility and authority of the pilot in command.
[...]
(b) In an in-flight emergency requiring immediate action, the pilot in
command may deviate from any rule of this part to the extent required to
meet that emergency.

There's a big difference between "disregard just about everything" and "to
the extent required".

In this case, the OP wasn't forced to do anything, he was just prevented
from doing one specific thing (entering Potomac Approach airspace). He had
choices short of declaring an emergency, and the controller was asking him
which of those he was going to pick. He could have asked to hold until the
weather got better (which is what he did) or until Potomac was able to work
him. Or he could have landed back at Hagarstown. Or perhaps Potomac would
have been willing to work him as far as Fredrick, which at least would have
gotten him a little closer to his destination.

You get to declare an emergency when the safety of the flight is at risk.
Being inconvenienced and ****ed off at ATC for giving you a bum clearance
isn't an emergency.

There's one thing that bothers me about the original posting.

"Mike Granby" wrote:
Now, I'm not happy, 'cos I know there's been cells appearing along that
route all PM, but I have little choice, so I take the SCAPE route.

That sounds like get-home-itis. Landing at Hagarstown was a possibility.
Sure, it would have sucked to go right back to where you took off from 10
minutes ago, but it was a possibility. If you're not happy with the
weather, don't go there. You make it sound like it was a choice between
heading to SCAPE and running out of fuel.