Just freehand the stones when on mountain. I have an old medium grit Swix one that I carry all the time and a rough stone for major edge damage.
If you can't keep a stone flat against the base edge then you need practice.
The chance of you dulling the edge is pretty slim as the stone would have to be balancing on the edge and stay like that consistently. It may happen by accident but not down the entire length of the ski. Just put more pressure towards the inside of the base edge instead of towards the outside and use your fore finger as a guide up the sidewall of the ski.
Messing with the base edge too much or uneccessarily, even with a guide, will cause you to increase the bevel if you put too much pressure on it to make edge contact.
The side edge is a different story. It is hard to freehand on the side even while the ski is in a vice, forget about on mountain on a picnic table.
On Mountain I would leave the side edge alone unless you have some serious hits that would effect your edgehold while carving or possibly ripping your jacket when transporting the skis.