They don’t need to test with a spent F9. They already did with raptor engines blasting a steel plate in mcgregor.
I am not saying they have to build something equivalent to the SLS/SST/Satun/Soyuz/Proton concrete pad infrastructure. Just put a 45°. Wedge, cone, pyramid, whatever shape works. Make everything out of steel if it works and is cheaper.
Since the Super Heavy and the Starship are lifted into place by the tower instead of being rolled already assembled or rolled and erected from horizontal, it would be far easier for Super Heavy to have an above ground diverter compared to other rocket systems because it is no longer necessary to build the artificial hill and ramp on which to roll it. But the diverter is still needed.
And wait years for the soil to settle.
Note that even in the test you linked the plate was angled. This is not done just to redirect heat but in order too redirect the sound: https://physics.byu.edu/docs/publication/4678
Reflecting that towards the engines is probably suboptimal.
There is a reason flame diverters are diverters instead of blockers. It's much much easier to just throw the exaust to the side instead of trying to fight it head on. Don't try to use force to block it, put a 45 degree angle and use geometry and physics to your advantage.
If they really want to experiment with unconventional solutions they could make a small scale test with a spent F9 and see if the idea is feasible. For what it's worth, even the F9 uses a flame diverter trench.
I get why they don't want to. It probably means modifying the tower too, probably to make it taller. But I foresee this shower head plate going badly.