pentax bodies, afaik, will disable the hotshoe when shutter is configured above sync speed (which is reasonable). hss is not a solution to your problem, neither are nd filters and aperture, the only solution is either faster sync speed or more powerful flash.(if you want fast sync on the cheap, get an old powershot and check what the chdk project can do for you
). if you want to do it with your pentax, i'm afraid the only solution is a more powerful flash (and this applies to canon and nikon too, with the exception of the cheaper nikons like the d40, which have electornic shutters, so sync speeds (arbitrarily) limited to 1/500 iirc)
ps: in case it's not clear: using a "slow-discharge" flash on full power above the sync speed doesn't solve the problem either: such a flash actually effectively limits your maximum sync speed to it's discharge speed, because otherwise you're not actually using it's full power, so you haven't achieved anything