If it's already got silkspan on it then most of the hard work is done, and you just need to put a bazzilion coats of dope on it.

Were I to get that plane in my hands and decide to recover it, I'd cut the silkspan out of the open bays, sand the plane smooth, then cover.  AFAIK the silkspan & dope that's already on there will do nothing but improve the finish of the plastic film.

But.

Don't be surprised at where St. Peter directs you when you pass away, you heinous sinner, you!

Just slap a couple of coats of clear dope on it and go fly.  Small holes and tears can be fixed with scrap silkspan and SIG Glue.  Don't worry about the pretty as once he gets the hang of what that thing in his hand is for, he will be flying circles around us.   

To answer your question directly, YES you can put iron on directly over silkspan. It works very well and you get the best of both worlds that way. The silkspan gives the wing rigidity, and the monokote ( iron on of whatever brand, I prefer Ultracote) gives the open bays puncture resistance and pretty too..
go for it, it will be easier for the most part to repair though on repairing it, I doubt you will be replacing any damaged silkspan,, ?