I am supposing that you want a solution which is fully programatic, so that the actual plot portion has a precise size (matching that of others, say). Otherwise you could use the right-click context menu to set the plot "manipulator" and zoom and pan by hand (thus creating the extra space for the well-placed annotation).
It might be possible to set up an easy-to-use procedure to do that fully programatically using the
I'll try and do that when I get computer access, perhaps this evening.
The idea is to increase the (viewport/bounding box) while zooming-out (scaling) by a corresponding factor so as the keep the actual plot size fixed. Then the plot could be positioned within the box by offsets (pan). If I recall the DocumentTools stuff allows these bits to be programmed.
I'll let you know if it works well for me, unless someone else codes it up first.
update: It turns out that I was mistakenly thinking of the way that zoom and pan works for 3D plots. I don't see how the above could be used for 2D plots, in which even manual zoom and pan affects the axis view. The suggestion to annotate by using plots:-textplot (which can be used to insert typeset math as well as text) seems like it may be the only simple approach.