One of the areas I'm interested in filming is between 4.5 and 5 miles from a very small general aviation airport (no commercial traffic). If I stay under 100' in this area, is it reasonable to 'skirt' the letter of the law in this case?
No doubt you'll get plenty of opinions on this, many of which will be along the lines of the regs are what they are and should be followed to the letter. And there is probably no small amount of wisdom in those opinions.
That being said, my own opinion as an ex-commercial pilot and current Phantom enthusiast is that if a fixed wing aircraft is below 100' while still 4.5 miles out from an airport, then both you and that pilot have way bigger things to worry about than the fact that your 3lb plastic quad copter is flying somewhere in that particular chunk of sky.
Now you should note that I said fixed wing. Though its rare, its not entirely unheard of to find rotorwing (i.e. helicopter) traffic down as low as 100' that far out from an airport. Your saving grace here is that down that low, they're probably not going to be moving all that fast and, giving that its a helicopter we're talking about, its going to make a fair amount of noise thus give you lots of warning that something's up in the neighborhood thereby allowing you to get your phantom on the ground long before its likely to be an issue.
There are a few notable and very important exceptions to the above statement. The first is if that particular area, less than 5 miles from an airport though it is, happens to be on a published military training route or otherwise someplace the military conducts operations regularly. Down low and balls to the wall fast is how those folks love to operate. Even if you hear them coming, you might not be able to get out of the way fast enough. Its a risk you don't want to take.
Another exception would be if there are regular glider operations anywhere in the area. You won't hear a glider coming and the glider pilots ability to take evasive maneuvers will be limited at best. And even though your phantom is very light, it could do a nasty job on the plexiglass windscreen of a typical glider and below 100' while presumably setting up for an off airport landing is not the time a glider pilot wants to have their windscreen cracked open and/or have a small remote quad copter, or pieces of it smack them in the face.
And the third exception would be skydivers. Skydivers under canopy do make a bit of noise as the canopy flaps in the wind. But its not a ton of noise and could easily get missed if the winds were right and you were concentrating on other things. But like the glider pilot, a skydivers ability to take evasive maneuvers will be extremely limited.