What you can probably do is use Sitecore's Rules Engine, where you can create (or use a pre-existing) Condition
to find out whether you are on a mobile device (to do this, you could use the Mobile Device Detector for instance - although I don't know if that module is supported on Sitecore 7).
Then, you can also use the Action
Set placeholder to value
.
You could also create your own Action instead, of course. All you'd have to do is get the RenderingReference
and simply change the placeholder like so:
var reference = new RenderingReference(this.RenderingID);
reference.Placeholder = "A";
More on this in the Rules Engine Cookbook
Update
I've written a blog post about how to get this done: Read it here.