I think http://www.screwfix.com/search?search=plasterboard+spring are about the strongest fixing you'll get for plasterboard if you don't have a nogging available. No idea of their load bearing ability though nor if it's the right way to do it.
The shower screen problem is,to me, a little different due to the waterproofing issues and also a little easier using a silicone sealer as "glue" due to the lesser load and larger bearing area.
I had considered the toggle solution as one possibility and am pleased to have some support for that option which I will try if I don't hear from anyone who has actual experience of a better/alternative way.
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