It likely will cause too much turbulence if you open the 3 way valve during pre-brew. The way around this on an archaic machine like mine is to kill the power to the pump by interrupting the signal to the pump solenoid (that's what I do). On a sophisticated machine like yours with a gear pump, you'll need to find a creative way to kill the pump with the solenoid open. Since you're likely plumbed, as well, things get even more tricky because even with the pump off, you still have line pressure hitting the puck any time the 3 way is open.
If Slayer uses a 3rd party pump controller, you could arguably interrupt the enable signal to the controller and kill the pump, but you'd still have the issue of line pressure pushing through the gear pump and 3 way. I'm not sure that this is actually possible without throwing a fault (Lukas, paging Lukas!), but it would be step 1.
If -and only if- you found a way to shut off the pump, you would then need to find a way to close off the line pressure from reaching the puck. This could be done by outboarding the needle valve (and replacing it with a less-precise unit that allows ergonomic adjustment from some relative max pre-brew flow to sealed) and closing it (again, only after shutting off the pump!) Or by installing a solenoid valve somewhere in the brew path that could only be closed when the pump was off...
Perhaps the easiest way to pull this off on a Slayer would be the opposite approach... rather than try to shut off the pump mid-shot, you could find a way to hold the 3-way open with the brew paddle in the off position:
•Imagine a DPDT relay with a switch on it that is powered off of the normally closed contacts on the paddle micro switch (paddle off equals power flowing to the switch.
•The first set of contacts in the relay has power on the common terminal and the normally open terminal connects to a normally open 2 way valve. When power is applied to this valve, it closes off the path from the inlet to the group.
•The second set of contacts in the relay has the solenoid hooked to the common terminal, the factory power wire hooked to the normally closed terminal and power applied to the normally open terminal.
•With the paddle off, power flows through the normally closed contacts of the micro switch to your new "soak" switch. When this switch is off, nothing happens. When this switch is on, power flows to the coil of the DPDT relay and sends power to the normally open solenoid valve, which closes it; power also flows to the 3 way solenoid and opens it.
Here's your blooming shot sequence:
1: Start your shot like normal in pre-brew and turn on your soak switch shortly thereafter; it won't do anything till step 2.
2: Sometime before drips form on your basket, bring the paddle back to the off position. This will simultaneously shut off the pump, shut off the water supply and hold the 3 way open for a bloom phase.
3: Once your bloom is "complete", bring the paddle back to pre-brew to build brew pressure.
4: Finally, bring the paddle to the brew position to pull the shot in earnest.
It sounds like a mess but really, it's only two new components and they can be installed outside of the machine and the mod would be 100% reversible. Since the machine is "in between shots" during the bloom phase, I dont think you'll be in any danger of unwanted fault codes or anything like that...