Despite that petition being denied, Part 47 97.309[1] is pretty clear that as long as the mode's "technical characteristics have been documented publicly" it's fair game, subject to 97.307(f) [2].

[1] https://www.law.cornell.edu/cfr/text/47/97.309

[2] https://www.law.cornell.edu/cfr/text/47/97.307#f