I don't see any adverse effects from feeding a snake on a random schedule, but I don't know if I would make it that wide of a range between feeding dates. I have on one or two occasions because of external circumstances. (ie: Our swamp cooler went out over the weekend and because we live in the desert the are still experiencing extreme heat I did not feed on a scheduled feeding day because I worried that the snakes would all regurge due to the ecess heat.)
I've also seen it mentioned here and there here on the forums that adult snakes that tend to put on weight when fed every 7 days are then placed on reduced meals at every 14 days.
It is best to keep in mind, if attempting a random feeding schedule like the one mentioned, the individual snake(s) habits and health. I'd also not recommend a feeding schedule like that for snakes that are under around a year to year and half in age - hatchlings and yearlings do a considerable amount of growing and they need their nutrition.
The other thing I'd recommend is to keep regular weight records of any snakes placed on a random feeding schedule to monitor any growth. This will give you an idea if the snake is loosing too much, staying at a steady weight, or gaining in small incriments (sp?).
If you are concerned about a snake getting enough exercise, I've heard of owners and breeders both giving their snake(s) tub time. Basically allowing the critter to swim around in about an inch or two of water in the bath tub or in a large tupperware for exercise.
Jenn