support is a given. suggestions will come in a flood, too, i expect. here's the first of what i'm sure will be many.
start picking out your existing quilting in the center of the quilt. AND I DO MEAN PICK. DON'T RIPIT. that's only good for frogs.
do only a little at a time - maybe a 12" square, dead center, first. TIE THAT SUCKER where the blocks join, plus one in the middle of each block. when that's tied nice and flat, pick out the machine quilting for two blocks out, all the way around the part that's tied. tie those blocks. keep working your way outward, smoothing an tying as you go.
you will accomplish two things: (1) the layers will be secured and flat and (2) you can either be satisfied with the ties and bind it, or machine quilt some (or all) of it again. either snip off the ties as you get to them or quilt around them.
if you already know you won't be happy unless you machine quilt the whole thing, then use the same system, but put safety pins where the ties would have gone. take them out as you machine quilt.