Yeah, I don't think it'd apply particularly well in this case. The uneven slope and variable surface of the 'runway', alone, would make it unreliable.
But still, just having any such performance/acceleration check in mind - on any unusual take-off - ought to give pause enough, you'd think.
If I was ever so adventurous as that chap, I'd try a solo take-off first and then assess if any more load was feasible and safe.
Maybe helped by another rule-of-thumb: For a given situation, for every 10% increase in T.O weight you need 20% more T.O distance.
I'm not sure how reliable that one is, either, but one could confirm it, for one's own a/c, experimentally by taking note of two max performance take-off runs (in identical conditions) one with, and one without a load - an instructor, say. The numbers could be extrapolated to roughly prove (or not) the rule, no?