Unbelievably huge, unqualified success!
We now have not just "Slayerlike" shots, but actual Slayer shots. And it's easy. And it's reversible (requires some modding... sort of... Actually just rerouting the existing plumbing, so no cutting. And for me, it was free, (because of some spare parts I have kept around over the years).
In a nutshell, I routed the brew line through the hot water valve, which is a needle valve. This means ALL of your brew comes through the water valve. If it is closed, NO water gets to the puck/group, no matter how hard or long you run the pump. To do a Slayer shot, you just crack open the water valve, start the pump right at full pressure and keep it running. After it fills the lines and the headspace above the puck, pressure will start to build, only just as with Slayer, it will build all the way and not stop at one or two bar, even though the actual flow is still slow. You will see the thickest stickiest gloopy gloppy espresso start coming out and then you can increase the flow by opening the knob (at whatever rate feels good to you). As the shot progresses, you can slow the flow by progressively closing the knob as you wish (which you cannot do on Slayer), and end the shot by turning off the pump or closing the valve all the way and then turning off the pump. My very first shot, with no practice and experimentation, was a masterpiece. I just used the same grind setting that I had been using for the last few weeks doing Breville-"Slayerlike" shots using PP55. I wish I had four hands and an LED espresso light and a fancy mirror for video'ing. You guys would die.
If you've got some dark beans and don't want to do a Slayer shot, just leave the water valve full open and it will work just like normal, with your pre infusion settings and all.
You will of course, no longer have use of your hot water knob for tea. For that, I have a 1500watt glass kettle with temperature selection. Well worth it if you drink tea, Aeropress, or pour over. Coincidentally, it is also a Breville. But I am not a shill for Breville. I have that kettle because my wife actually wanted it and picked it out completely independently from me and my input.
Full credit to Jake_G for this idea. It was not mine. Jake does not even have a BDB. But he has a big brain.
Words cannot describe how elated I am over this.