Great explanation. The only comment is that Auto FP works off camera with the pocket wizard mini / flex combo, but this will cost more than the flash.
If I'm not mistaken it also works using CLS and the pop-up flash as a commander. This is how I usually use an off camera flash. It's been a while since I've used FP but I'm pretty sure that's how I used it...
This could be the solution the OP is looking for. It still depeneds on the situation and whether the flash in FP sync can give enough light for the shutter speed you need.
As much as I use a flash (almost always usually off camera) I would most likely choose other options before that.
Sounds like the background is much brighter than the subject i.e. the light isn't great for the subject. So I'd look for a different spot, different time, etc.
If I really want to shoot that specific moment, I'd look for a different angle, or possibly just compromise with an overexposed background, for example.
I'd likely not think it's critical to use an f/.28 aperture. At least raise it to f/3.2 which is more or less meaningless for the purpose of DOF (very small difference) or even less.
I might accept a compromise between subject and background exposure and use software to improve it as much as possible.
I'd use a flash, either at regular sync speed with a smaller aperture or use FP, depending on what each option gives me in the situation and none of the other options give something I can accept in the situation (in reality, extremely rare).
Of course there's the option of using a filter with regular speed sync, which is entirely possible (I just don't do that).
