It shouldn't be an issue should it? My Hasselblad has the leaf shutter in the lens and not a focal plane... And it only has a max of 1/500, and if I were shooting indoors I probably wouldn't be that high anyway.
Okay, you didn't say you we're using a Hassy before. I was just concerned because some cameras can sync as low as 1/60, like some focal-plane shutter SLRs. Shutters in d*g*tals seem to run roughly 1/200, and high sync speeds are useful for blocking out the ambient and enabling sufficiently open apertures to get shallow depth of field for portraits.
A really quick way to bake this kind of info into your brain... is stick a polaroid back on and grab a notepad. There's something about taking a shot, examining it, coming up with what seems the correct next step, and trying it out instantly... vs. shooting a roll and processing and trying to suss out notes for every frame. In the pre-digital days it was a dynamite way to learn some more advanced stuff very quickly or test your crazier ideas.
There's still 2 decent flavors of pack film available after all... and peeling those things always brings back memories (for me anyway).
EDIT: you can likely get a hassy polaroid back for next to nothing thee days, if you don't have one...
You can reduce the output by using a layer or two of lens tissue over the sub flash. Main flash too if you wish. Since you're dealing with guide numbers, there's a bit of testing to be done. The gn can get you close, but it' a guide not actual.
John's tissue idea is really useful. When I'm at a bar with the mrs., I'm always tearing off little strips of bar napkin and licking the edge to cover the flash on her phone - if you curve 'em enough you get instant soft-box (OK, she's a always posting pics of dinner on facebook, it's not as effective for group shots but makes closeups really pretty...)
If you bother to read the Original Post in this thread, you would remember that this is a project to use manual flash. On purpose. Maybe the OP will end up learning something, which is not really a bad thing.
"She's always out making pictures, She's always out making scenes.
She's always out the window, When it comes to making Dreams.
It's all mixed up, It's all mixed up, It's all mixed up."