The difficulty of staging single-to-cluster is that you have deliver enough ignition 'stuff' to each upper stage nozzle to ensure a light-off. You probably do not get enough lower stage blow-through (hot gas and particles) to fill much volume and light too many motors, or else all the rest of us with single-to-single staging would be getting incinerated! So to make single-to-cluster work reliably, I am guessing you would have to design some sort of interstage ducting to control the blow-through, contain the pressure and not allow much expansion, split the flow, and re-direct it to all your upper stage nozzles. Most model rocket building materials are not going to stand up to much of that kind of abuse (temperature, pressure, erosive flow, etc).
I won't say it can't be done, but I have never seen (or heard of) anyone doing a single-to-cluster. We want to learn too, so if you pull this off, please let us hear about it!