Depending on how much power you want to run, & how much boost, you won't need to retard timing. I ran 7psi on a turbo without retarding timing, & it worked fine. But this was on a lower compression ratio 8.5:1. But you don't need to retard timing as much with an SC as the EGT's and cylinder temps won't be as high as a turbo.

What you are suggesting is possible, however I believe there are simpler ways to retard timing, but its not advised. Firstly, you would need a pulse frequency modifying tool, that can take the frequency and adjust it, they are available from the Jaycar website as a kit. But the torque limiter is based on speed, so the signal you would have to modify is the speedo signal, and you would need to modify it to read below around 20mph. So you can build a voltage operated switch that will turn on at a preset voltage, then use a MAP sensor to give that voltage. You would just need to find the frequency of the VSS at low speed, and transfer the same signal to the ecu.

The simpler way would be to just use a megajolt & piggy back the standard ECU, and have it retard the timing on boost, then you have complete control over spark timing!

But, if you know how to tune an engine, you will know that the more you retard spark timing, the more fuel is required!! So the more you retard timing, the leaner the mixture becomes, not good on a high compression engine with boost.

So my advice would be either keep it below 5psi (5psi on an SC is more power then you might think), and use standard management, or go for a 5th injector & piggyback ecu to retard timing and give you the retard and extra fuel you need.