I encountered a similar situation when mounting my VP150 with FMB to sheet rock with no stud. In theory some compression-spreader-type dry wall anchors (aka hollow wall anchors) will hold it. However this varies depending on dry wall thickness, distance from stud, and probably environmental factors like humidity. Also the heavier-duty anchors may not be compatible with FMB -- screw diameter may be too large for the FMB screw holes.
Make sure you don't use a simple plastic expansion anchor. Use some type of spreader anchor (toggle bolt, molly, etc) that expands behind the dry wall. For details see
http://www.naturalhandyman.com/iip/infanchor/infanchor.shtm
My HDTV was beneath the VP150, so I decided not to risk it. My options were concealing a board behind the VP150 that spanned to the closest studs, then mount the VP150 to the board, or shifting the entire speaker, TV, etc. to the right 8" to hit the stud (which I ultimately did).
In your case you can't move L/R. While the right type of hollow wall anchor could probably hold an M22 via FMB, in general I'd suggest using a board behind the speaker to distribute the load, as you described. I'd still use spreader type hollow wall anchors to hold the board. It's better safe than sorry in situations like this.
