The Collaborative International Dictionary of English v.0.48:

Treasonable \Trea"son*a*ble\, a.
   Pertaining to treason; consisting of treason; involving the
   crime of treason, or partaking of its guilt.
   [1913 Webster]

         Most men's heads had been intoxicated with imaginations
         of plots and treasonable practices.      --Clarendon.
   [1913 Webster]

   Syn: Treacherous; traitorous; perfidious; insidious.
        [1913 Webster] --Trea"son*a*ble*ness, n. --
        Trea"son*a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

treasonable
    adj 1: having the character of, or characteristic of, a traitor;
           "the faithless Benedict Arnold"; "a lying traitorous
           insurrectionist" [syn: faithless, traitorous,
           unfaithful, treasonable, treasonous]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "treasonable":
   Judas-like, Trojan-horse, apostate, betraying, breakaway,
   collaborative, degenerate, disloyal, double-crossing, extreme,
   extremistic, factious, faithless, fifth-column, insurgent,
   insurrectionary, mutineering, mutinous, quisling, quislingistic,
   rebel, rebellious, recreant, renegade, revolutional, revolutionary,
   riotous, seditionary, seditious, subversive, tergiversant,
   tergiversating, traitorous, treasonous, turbulent, turncoat,
   two-timing