I don’t have a Holland grill, but after reading about it last year, I experimented with my old gas grill. This is what I did to simulate the grill. I removed all the lava rocks off of the lower grate and place a heavy-duty foil cookie sheet on the grate. Then I placed few of the lava rocks on top of the foil sheet to weigh it down. Finally, I replaced the cooking grates and fired up the grill as usual. I cooked all types of food on that grill, the only thing I notices, was that the grill never got hot enough to sear a steak. And I also noticed that it was cooking a little slower than my open flame grill. This was not necessarily a bad thing. If you’re trying to slow cook something that was one way to do it (other than the indirect method). It did grill perfect hamburgers though and without any flare-ups.
This may not be scientific or an accurate experiment, but it allowed me to pick the open flame method over this. You’ll have to judge for yourself.