Just found some lichen in a plastic bag I treated about 25 years ago - still spongy and soft!
Here's how I treated that stuff: I bought lichen in fairly big lumps, then I sorted out all the coarse branches, pine needles and other stuff. Never found a rattlesnake there, tough! (At least in the US where lichen can be found in the woods these slitherin' critters enjoy their stay in there - so take care when 'harvesting' lichen! Never found one, Vic?)
Back to treatment: I put the stuff into a pot with a mixture of about 2/3 water + 1/3 glycerine. Then I heated the 'soup' for 2 hours to about 50° C (about 120° F). Open the windows - it STINKS!
The Glycerine replaces the water in the cells. And since it almost doesn't evaporate, the lichen stays soft for a long while.
Of course, after years in the open air it becomes brittle just the same - but for background scenery, where it isn't touched, this is ok. (My sample was in an airtight plastic bag, so it remained soft.)
Tyson is right about fading. The screaming red 'autumn' lichen bushes from Faller were worst. They bleached out to a hideous dirty gray-brown-orange. I always hated it.
However, IMHO when a background forest is covered with dark green Woodland Scenics 'leaves', the grayish lichen branches don't disturb.
About zip texturing:
IMHO together with the Hardshell method still a fine technique for landscaping. I plan to use this method on my new layout again. It's fast and doesn't look bad, especially on nearly vertical cliffs. Of course, later you can come back and superdetail the landscape.