An example on a foam obelisk I built last year (this was with a brush application):
As long as you're not applying 20 coats of the paint/mix, you shouldn't be able to tell weight-wise that you've got the mix in there.
I used it on my crypt, one coat of the paint with the mortar mix in it, and it really just goes on like normal paint. I have used both roller and brush, and the roller is easier since you get brush marks and have to do some re-brushing or cross hatching as you're painting in order to not get obvious trails in the mixture.
It works both as texture and as a sealant - any latex would be a sealant, but the mortar seemed to add a better seam coverage in addition to the texture. You've got limits on how much to add before it becomes goop, and I always used an airtight container to store any excess (it will pull all the moisture out of the paint and turn into a giant block of gray stuff if you don't)
If you've got some mortar mix sitting around, give it a try on some scrap foam and see if you like how it looks. Couldn't hurt, and at worse you'll just have to get the sand...
