Some of the bag mounts I've seen use a plate that fits between the fender rail and the backrest mount. If a 3/16" or 1/4" plate is used then 3/16" or 1/4" has to be trimmed away from the existing bolt spacers that hold the backrest mount off of the fender rail, so everything fits back up correctly. This plate should have bolt holes that match up with the 2 front bolts holding your backrest support on. The plate may cover the rearmost bolt head in the fender rail if you install the plate next to (against, touching) the backrest support bracket. Your bag mount then gets attached to the plate, under the backrest mount. Of course, another plate is required for the other side. Look at a set of Leatherlykes for an example of what I'm trying to describe here.
This is the order, from the outside in, towards the tire:
Bolt head
Backrest Bracket
Saddlebag plate
Spacer
Fender rail
Fender
Inner fender support bracket
Tire
Hope this makes some sense.
EDIT: You can also mount a plate or your saddlebag brackets directly to the ouside of the backrest support bracket using the two front fender rail bolts (you'll need longer bolts - they're 10MM x 1.25 - NOT 1.50). But I do not like the look as much as I previously described.
Have fun - I just completed my bag mount mods - and love 'em.
Todd