I have heard of silnet on the floor of tents to help with pads slipping but can the idea be applied to the inside floor of a bivy? Can it be applied to the bottom side too to help with sliding on a polycro ground sheet?

Generally, yes. As long as you apply the silicone to a silocinzed or bare woven material it will stick. I note that for stripes, they should be troweled out, but may still peel. Thinner is better than thicker. A trowel cut from a plastic milk jug works well. Contrary to what you might think, use it to scrape away any excess, not spread it.

For a bivvy or tent floor, simply diluting it well with mineral spirits/White Gas brushing it well onto the whole surface works: about 15 to 1, MS/Caulk ratio. Painting the whole bottom will work about as well as just applying stripes/dots close enough to work.

Besides dirt/dust on the fabric, the thickness of the silicone seems to be key to keeping the stuff from peeling. Adhesion between the substrate and the silicone needs to be higher than adhesion in the silicone. The result is generally no peeling. If applied too thick, it can peel.

I use this same technique on canopies when they are stretched and leaking, or, misting, too.

I applied diluted silicone to both sides of bivy bottom - for water-proofness

Bottom started peeling after a while where it abrades against the ground

I probably didn't dilute it as much as James suggested so that might have worked better

But the fact that it peeled wasn't a big deal, except it was a little messy. I finally just rubbed it all off

Inside was fine for many trips

Does silnet "stick" to the polycro enough to make it worth it? I also wonder if dirt will coat the silicone and eliminate the stickiness like coating dough with more flour.

I have not tried silicone on polycro, but it didn't stick very good to non-silicone fabrics that I've tried

I've tried it on polyester thread and that seems to work fairly good