I've used silk flowers on cake dummies, but I have reservations about using them on a real cake due to the cleanliness of the silk flowers.  You don't know how they have been handled or how clean they are, so I wouldn't want them to come into direct contact with a buttercream or fondant cake.  Is there a way the underside or other parts that will be touching the cake they can be coated with something food safe?  

is there any way they could be washed?

Even if they were washed, they're not made from food-safe materials, so they shouldn't be touching the cake.  I was thinking there was something I could spray or coat on the back side that would be food-safe.

Paint them with melted white chocolate, and when it cools off it will make a little barrier between the flower and the cake.

if you are that sensitive to them not being safe why try so hard to use them anyway -- putting melted choco is a great idea but what if someone eats that -- i mean non-edibles should not be on our food i guess -- wouldn't put them on our mashed potatoes right -- but i don't think a washed plastic stem is toxic unless there's a wire in there -- or the coloring in the material is unsafe -- so if you are that sensitive i don't know how you could still use them --

you could still put the plastic stem into a plastic drinking straw and insert that -- could coat with costumeczar's melted choco first -- bam

edited to say but the chocolate could crack off and be ingested -- i would wash them off -- be sure there's no dust on them and use them in the same sense as we use other plastics and toys on cakes -- you could also coat them with a soy wax but flowers are easy to do in gum paste too or place them between tiers not on the cake

A very popular franchise cake company, Nothing Bundt Cakes, uses tissue paper pom-poms and silk flowers directing on top of their bundt cakes all the time.   Just google, Nothing Bundt Cakes and then click on images.

Personally, I don't see a problem as long as the silk flowers are clean.

That is a good idea, coating the flowers and putting the stem in a straw seems to be the way to go.