ah man, you're taking the hardest way to grow weed, guerilla, and making it even harder! alright then, if that's what you must do, you're going to have to simulate night somehow to get them to go into flower - which means you'll have to be at the site at least once a day.
The first way is what everybody has mentioned already - you can move the plants into a darkened structure or you can cover the immobile plants. I'm not even going to start with suggestions on how to construct a backcountry blackout room. You either have to put the plants to bed at night and pull them back out again in the morning, or you have to cover them at least until night falls - I see a lot of people reporting success with pulling their tarps before dusk, then removing them a few hours later - this would cut your trips out there to once a day.
I would also shoot for indica dominant strains. Once you have been blacking them out for 3-4 weeks, they should be far enough along that they will continue flowering even after you stop blacking them out. Sativas and sat dominants have a tendency to revert to vegetative growth if their light cycle gets screwed with too much. good luck! let us know how it goes...