Actually, yes, there are two ways to do it...depending on which vintage FlareR you have/get.
The original FlareR came with what we call the Sync mounting bracket, which can be hacked to mount directly to the rear reflector mount that comes with the bike. In the pic below, the Sync bracket is shown (just
barely) stretching around the post with a Flare1 attached and a hacked bracket is shown just to the left (below) that, attached to the reflector mount:
The Sync bracket is also available separately as PN 418921 (MSRP ~$7). I have several of these mounts, so I can leave my SC and Emonda front&rear light-ready at all times and just pop the lights on and off depending on what I'm riding.
The current FlareR ships with a different band that isn't hackable like the Sync was. You can, however, get PN 418932 from your Trek dealer, which is a rack/chainstay light mount compatible with the FlareR. MSRP is about $2, and it can be hacked in a similar manner to the Sync mount such that you're bolting directly to that same reflector mount...just in a different way.
Here's another pic...slightly different bracket than 418932, but I think it gets the idea across:
Carl Matson