It may be safe, but IMO you should NOT use BH 209 in a sidelock I love the stuff in my inlines but there are too many negatives to use it in a sidehammer.
1) The grains are all the same size (extruded) and probably won't fall into the sidelock well
2) BH 209 is hard to ignite and you need a strong primer, but not sure it will ignite unless directly under the primer.
3) BH 209 creates more blowback that any other powder I have used, so if you do this WEAR EYE PROECTION. And expect your hammer to be at 1/2 cock or ever full cock after the shot.

I just can't see any reason to use it. I have used pyrodex or 777 in my sidelocks, and yes, you have to swab between shots and yes you have to clean up at the end of the day. BH 209 works great in most inlines, but I would never use it in a sidelock. JU2C