Photo flo would thin it out but I dont recall it helping much as far as lubrication goes. Glycerin is all over the place, $3-4 will buy a jug. Cake decorators use it to preserve to color of their frosting. It's handy stuff.
I believe they still make it. If it's not available at some point, i'll be using jetdri. Or tween.
I used to add photo-flo to my cyanotype. It never helped me much. My method for that was similar, except I use a foam brush. A nice wide one.
I still mix the glycerine in, 1 - 4 drops depending on the surface of the paper, it it's watercolor, etc.
Strathmore400 is my saving grace as far as cyanotypes go. Those little 4x6" sketchbooks are perfect for it.
Unfortunately the paper has been an iffy choice for van dykes.. I tried sizing with knox gelative, arroroot starch, cornstarch... then I heard of Cranes Kid Finish Ecru White..
Phil's suggestions make a lot of sense to me. Would photo-flo work, too to break the surface tension of the solution. I don't even know if Photo-Flo is even still available.