I can't figure out how I could properly get my own mod item supporting facade without extends Pipe stuff. So I was wondering if it would be posible to add a simply interface in the api like :
public interface IFacadeSupport {
public boolean canPlaceFacade(ForgeDirection direction);
}

No. An API already exists. Use the IMC for it:

https://github.com/BuildCraft/BuildCraft/blob/master/common/buildcraft/BuildCraftTransport.java#L446

I though the IMC is for adding new facade textures, not for allowing facades to be placed on custom blocks.

Ah, right, other way around.

I think this can be done, but you do realize that you would have to handle all the facade rendering yourself...

It can be done. AE does it

Facades are dumb items as far as I know, the pipe does all the fancy stuff.