Don't know nuttin about DC.. I have been known to fly this Great Lakes
area... Your direct route will be across Canada (A foreign nation,
legally - and a bit strange, factually) but I digress - anyway,
"theoretically" you have to file an International Flight Plan to enter
and leave foreign airspace (unlike the old days where we totally
ignored such niceties).. Now, I wouldn't swear on a stack of bibles
that since 9/11 I have not accidently clipped a corner of Ontario on
occasion (koff) on my way to Cleveland but I haven't merrily flown the
entire E-W length of Ontario - as I used to do - without bothering to
talk to the 'authorities'...

So, the question in my mind is if you are planning a GPS direct from
Mackinac to DC area, what are you planning to do about the
international portion of your flight? I suggest giving your local FSDO
a call and see if simply filing a flight plan will cover your butt for
the International portion of the trip...
Even if it does (which I doubt) you still have the issue of re-entering
the USA if you are forced to land in Canada for any reason - weather -
sickness - engine - USA airspace is suddenly closed - etc... Canadian
authorities will be sweet as pie, but the a**holes on our side of the
border seem to get their rocks off harassing american citizens... A
young fella, who worked for me at the the time, took his 17 year old
girlfriend - blond, blue eyed, picture pretty, sweet as pie, all
american girl - on a tour of Ontario over the 4th of July weekend...
When they turned up at Port Huron to get home, guess what - she didn't
have a passport or birth certificate with her, they refused to let her
in - period! It took seven hours of phone calls to The Father -
Michigan Senator - US Senator - Chairman of the Oversight Committee
of US Customs - who phoned and requested the Customs Officer to let
her in - and finally had to resort to threats to get him to comply...
If it hadn't been that the Father was a direct acquaintance of a State
Senator (and had his home phone number on a holiday night) who was a
personal friend of a US Senator (ditto), I don't know how many days it
would have taken to get her re-entry into the USA... So, do your
homework..

Suggestion: From Mackinac fly GPS South to a point 5 miles East of
Detroit City Airport (DET), air file a flight plan to fly VOR direct
Windsor (YQG) direct Sandusky (SKY), and once back in USA airspace
cancel the flight plan... You are in Canadian airspace for about 20
minutes... Lots of planes coming and going that route... This is the
route I flew when I accidently clipped Canadian airspace (ahem)...

The alternate plan is to fly to the point just East of Detroit CIty
Airport, then follow the international border down the centerline of
the Detroit River, down low to stay out of the Detroit B air space,
then follow the international border across Lake Erie to Ohio and then
to DC... Low hassle, nice view in good weather...

cheers ... denny