VVC, I imagine one could apply the No Rez prior to assembly, but I wouldn't encourage it for a first build. I think it would add complexity that would be better saved for later. Some larger cabinets have inaccessible areas once box is assembled...I suspect that's what you've seen. The way I designed this, it's better to do all assembly in one fell swoop just like I show in video. As Danny pointed out, No Rez and crossover are easily installed later.
I'd encourage a couple of things. Do a dry run to get the rhythm and sequence of assembly down before adding glue to the equation. Run screws in and out of pilot hole for crossover board before cabinet assembly. That way when you go to put the board in place inside box, the threads will already be cut and make insertion that much easier.
Bear in mind that the No Rez doesn't have to be in large continuous pieces. The effectiveness will be the same if you installed a side in, say, four quadrants. It also doesn't have to be pretty to be effective nor does it have to cover 100% of a panel. It's about how it damps the panel resonance, not so much continuous coverage. Make sure surface is free of dust, and be deliberate when you place it, the adhesive is really aggressive which makes repositioning a bunch harder. I do dry runs here as well, to make sure what I've cut fits before pulling release paper off.
