There are several ways of flashing paper. Some even use a device sold by RH Designs. I simply remove the negative carrier from my enlarger and use the enlarger to flash for a time that I have tested and found to be just below exposure threshold. This will depend on how much light leakage you get out of the enlarger with the carrier removed. I don't have much from the Durst. I don't use any diffusion device at all in my preflash work. Since enlarger bulb wattages vary, it is difficult to tell you what times may or may not work for you. I typically find that at F16 and a grade three filter pack in my Durst at 8X10 enlargement that the preflash time will run about 1.6 seconds on JandC Polywarmtone Classic. Other papers are faster then this paper. I would preflash Oriental for 1.2 seconds.
Insofar as preflashing and split grade printing, yes it would work...I suppose...even though I haven't done it. I typically preflash at a single contrast grade.