Just thinking out loud and brain storming:
If the terrain (percentage split between ascends and descends, technical difficulty, and with the right number of laps), there might be a way to artificially create an "auto leveller" to give the participants freedom to use whatever spec they want, and however hard they like to ride. Bigger batteries are heavier, higher output motors require bigger batteries, and turbo levels will drain battery charge much quicker. If no throttles and battery changes are permitted, then a rider would have to:
a) Go flat out and leave everyone behind, but will have to ride the last few laps with no power;
b) Conserve battery and play strategically, then blast at the end laps;
c) Go big with motors and batteries but suffer the handling consequences of a heavier bike through the tight or challenging obstacles, deep water crossings (yes you heard me right!), and technical sections. Rules require to use only bicycle drivetrains and chains (no repairs or replacements), so overpowered beasts would risk snapping them;
d) Go with a super light bike setup but will need to supplement where it's lacking, with human power.
It's a big ask I know, but parity without policing might be the only practical way forward with eMTB racing. Design tracks that reward standard eBike specs but penalises thousand watt monsters. So if the track was designed properly - it might just be doable.
What got me thinking about the possibility is a YouTube video I saw of Jeff Kendall-Weed going toe to toe with a KTM Enduro motorbike. Motor bike wins obviously but surprisingly, on admittedly only a few sections - his eMTB wasn't outclassed as much as I expected it to be.