Scotty will likely remember the issues I had trying to find a way to get checkboxes to output something besides "on" or nothing at all. Well, I finally did it!

Use checkboxes just like Scotty explains in the documentation. Now, here’s the slight difference...
Normally, WLPE will return either on or a "blank" value. This hack I created takes advantage of it by having the filename named to something weird like png2 (that no program would
ever use). Now, that’s gonna obviously give a 404 error, because it’s pointing to a file that doesn’t exist, and shouldn’t exist.
Before you do the next step, create an onoff.png (like a checkmark) and off.png (1x1 completely transparent) file --- you’ll need this later.
That’s where the magic of .htaccess comes into play. Add this to your .htaccess file in the root of where you installed MODx CMS...
Options +FollowSymlinks
RewriteEngine on
RewriteRule ^(.*)\.png2$ $1off.png [nc]
Note that the [r=301,nc] will not work (dunno why) --- but it’s not like Google’s gonna search for this.
If you did everything correctly on.png2 will redirect to onoff.png, and the "blank" .png2 will redirect to off.png. It’s a somwhat
crude hack, but it works w/out the user having to do anything.
(For those who were wondering, getting to this point took almost four hours total...tested almost 20 different ways w/ failures before getting to this idea that worked. xd)