Lol, this would be a great time for local or dragon or ethan et al to stop by and comment on the basic theory of broadband absorption I have described.
But I do believe this is correct, and I do believe the conventional wisdom given by erskine and pretty much everyone else on avs over the years just doesn't make sense. I can assume only that either I am off my rocker, or simply that no one has stopped to think about what is going on for a moment.
The worst case that I think you might encounter is having too good of a broadband absorber. Its likely that the carpet will normalize the aabsorption to what a non trap riser might do in the upper frequencies. In the low midrange and down, it is possible that you'll get too much absorption if say the entire riser is open grating. A facing membrane over portions could vary that. Or you could just membrane all the grating. I dont have handy measurements of the absorption, reflection and transmission spectra for typical carpet and pad. That would help. And I guess its possible to be overly absorptive at bass frequencies but I think unlikely.
Optimally, you would build the riser and measure before final carpeting. Any indication of over absorption could be dealt with by simply "plugging up holes" to one degree or another.