You could probably make a small half-gallon batch of wort and use either the Belle Saison or Wyeast 3711 yeasts, and once fermentation is going strong, combine this with your existing tripel. Either of those yeasts (they are the same) will ferment pretty much anything down to about 1.000-1.005. You'll have to be patient, though... it can take up to 3-4 weeks for this yeast to complete the fermentation.
Good luck.