Hi @PaulaD,
I haven't been able to find anything that says yes or no in regard to Hardieflex, but Villaboard, which is made by James Hardie and is extremely similar to Hardieflex, can be wallpapered directly over the top without any paint.
The only issue I can foresee is that in an uninsulated application on the external wall of a structure, the Hardieflex will be prone to a lot of changes in temperature, which will certainly have some effect on the adhesion of the wallpaper paste over time. This is certainly something to consider in deciding whether to go ahead or not.
I suppose it comes down to how long you want the wallpaper to last. If it is intended to give the chicken coop a good look for a while then go right ahead, but if you wanted it to last 20+ years, paint or even tiles might be a better choice.
Let me know what you think.
Jacob
