Size variation in a Imagemagick montage

I try to have random space between my images in an Imagemagick montage.
My original script is:

montage images/* -mode Concatenate -tile 12x -geometry +10+10 -background white tile.png

…and I need something like:

montage images/* -mode Concatenate -tile 12x -geometry +($RANDOM%200+0)+($RANDOM%200+0) -background white tile.png

Is this possible?