Reload?
Oh, man, that's easy if you're a reloader. You can take many rifle cartridges subsonic pretty easily - a few can be finicky when you get way down to 700 fps and lower, because tiny loads in big hulls can be erratic sometimes with certain powder combinations. I'm sticking with the theory that igniting a very small charge in a very large rifle cartridge is where you run into trouble.
As Doc Winslow pointed out, if you're playing around with low powder charges on your own, getting a bullet stuck in your barrel is annoying if you notice it - and
dangerous if you
DON'T notice it.
In general though, you can either use Unique or one of the powders really developed for the cast lead boys (like 5744) and run 'em up to 1050 fps or so. I pick 1050 because when you're looking at "subsonic," you have to consider the deviations in velocity from one cartridge to the next, you want EVERY shot to be subsonic - so you have to factor the standard deviation as the error margin.
I've had limited experience with Trail Boss, and it's been good, but I haven't loaded the cartridge in question.
If you have a particular bullet and powder in mind, you might find one of us nice guys in the reloader's forum to run it through our ballistic software to get you in the right ballpark.