I understand your line of thinking. It seemed impossible to utilize the back straps on my setup, the outboard positioning just didn't seem right. I'm finally in the back straps in their outboard position. Here were my mistakes.
1. Position (Stand upright): When learning, I was trying to attack the wind much like a traditional windsurfer... leaning over and weighting in the heals. This led to multiple failures. You need to stand further upright, even when in the rear footstrap. Remember to lock out your rear foot/heal to keep the board steady.
2. Wind strength & angle: There was a video done here in the US that discusses the idea of not using rear straps at all for a while.
(Go to 4:24). This will allow you to move around the rear of the board and find the right position. You'll find in lighter winds that your rear foot will be more in the center of the board - or you'll find that the more downwind you go, your rear foot will be in the center. Since most start out in lighter winds for foiling, the rear strap can put too much pressure on the windward rail, therefore rounding up the board into the wind. I too got frustrated and was wishing for more center based strap positions. when the wind is stronger, I find the back strap with my foot.
I like you're line of thinking. We have a few that have done similar mods on their boards. For foiling, just start sailing with out the rear straps and when you get better in sustained flights, note where your rear foot is. Once I started going faster, I found myself in the right position to put my back straps back on. I may get the half straps like the video has... but in all seriousness, you don't really need the rear straps on a foil board.
