Good luck. This would be fun to see. Consider the following if your project makes it far enough along:

Early on in the War, Washington was eager for a fight. However, he learned quickly that the way to win the war was to preserve the army and to only fight when considerable advantages were held. Now consider the unique position of the British; they employed massive numbers of troops and attempted to win by taking control of major cities while seeking out the American army.

The only way for the continentals to lose was for the continentals to lose their precious army. Washington even considered moving as far as east as the ohio valley to ensure the army was preserved. As long as the army existed, then loyalty to the cause remained high enough against the British to sustain the new recruits joining the army. After all, the continental enlistment was very short, so loyalty to the cause was necessary to encourage new enlistees, and the reare reenlistee. However, for the British to control a city enough to squelch rebel talk, a large garrison was required. There were exceptions of course, such as New York, which was mostly loyalist despite having a significant number of rebel sympathizers.

So this can be easily simulated by always requiring a large number of british in a town in order for it to remain in control of the british faction. This hinders their mobility and forces them to send out detachments to search for the continentals. Once the british feel confident that they have located the continental army, they would leave the city. This of course would cause it to convert right back over to rebel hands.

Furthermore, the imposed rebel tendency of cities without garrisons should be eliminated with the demise of Washington's continental army. Washington, in large part, became the cause himself, sustaining it through his fierce tenacity and his belief that he was doing the work of "providence" as he called it. Some historians have argued that Washington recognized this time as momentous as it was, and that he realized that history would remember him by his actions. Thus, he hid much of his own emotions and sought to provide history with a vision of himself that he decided upon. Hence, terms like the "American Sphynx", the stoic, etc. He was referred to as "His Excellency" as both an insult to the King and as a recognition of his place as "American nobility", a nobility that is earned, rather than inherited or granted.

Simply put, the mechanics of the campaign map are essentially this:
(1) Washington and his Army are the cause. Elimination of this army should bring the continental chances of victory to almost nil. If this were barbarian invasion, you would easily be able to sustain washington and his army thanks to "hoarding", but we shall see for M2TW.
(2) For the british to retain a city (with the exception of New York), they must occupy it with a full stack. A reduction in forces should result in sustained city rebellions of minutemen and militia; not large forces, but enough to continually cause heartache for the british faction. The death of Washington ends these local uprisings.