Installing a new trim kit on a Kohler cartridge, and the new piece has no mechanical attachment component. No screw holes or clips to hold it to the cartridge. I checked the instructions a couple times, and it only uses the silicone to hold it in place.

The old cartridge sits about 1/4" proud of the tile, and I don't want to rely on the silicone to hold the trim in place while also pushing the cartridge back. I'm thinking of fishing a zip tie through the cartridge plate around whatever of the rough in is there to draw it back a bit. Is there a problem with this, or is there a better way?

Pics below

There should be a foam gasket included with the new hardware that may help you out. If not, get a few to double as a shim that fills in the gap. At the bottom of the housing there also should be a weep hole if you're worried about water getting behind everything.

Yep, have both of those accounted for. Still very odd that there's no true attachment point on this piece.

I ended up ordering some stainless hose clamp to run through there. Should fit the bill nicely, and I have more confidence in that than a zip tie

My bad, I missed that part. That is weird though. What about a few set screws on the outside of the round trim piece? It looks like you may have enough meat to grab some of the cartridge with it sticking out. Maybe too much of an effort?

Cartridge should be screwed to a 2x4 behind it. The handle has a screw that holds it into place are you sure it does not pull new assembly against tile?