Not having the coins for a surface grinder, I tried this affordable alternative today and I'm happy to say "That'll do, pig".
I bought a Dewalt cup grinding wheel and made an arbor for it to mount in my mill and it works.
I spent a little time surfacing this piece of mild steel and while it doesn't look like a high dollar surface grind finish (it's a Blanchard grinder after all and they never do) it did produce a surface that's as flat and smooth as I could hope for and it will work just fine when I need things to slide over each other smoothly and accurately. It's a better surface that I've ever milled with my carbide surfacing cutter.
Everything on the mill must be covered to catch as much grit as possible but plan on spending some time afterwards with an air nozzle and a brush, and make double certain that all of your covers will stay in place as the table travels and watch them closely anyway. I had a bit of trouble getting my cooling mist completely under the wheel's surface and so had a couple of spots that are heat discolored but I think I can solve that issue with more practice using it.
That test piece (which has had only soap and water cleaning after grinding) is absolutely smooth and flat as I run my finger nail across it so I know it will work for my purposes. All in I think I've got less than $30 in this experiment and I've added another tool to my repertoire. If you need a surface grinder and can't afford one, this technique is worth trying.