1. I am with Bill, in the don't-do-what-is-easy-and-available-elsewhere category, and certainly don't bother consuming engineering resources to do so. I don't agree with the contemporary mindset of trying to get the One Tool to do everything: the whole idea of efficiency, modularity, integration, etc that allows us to work productively, is to Use the Right Tool for the Job, which means being proficient in a small number of really good purpose-built tools, not being a master of One Tool.
2. Separate point. I do not see the problem in doing the requested task (gray-scaling out most or all of a page, in order to highlight a small section) in OG. I often do that. No enhancements or special requests necessary. I use the inverse of a balloon or call-out, with my changes identified in the left margin (that's what people who are used to Pages know and love). There's all kinds of sexy things you can do, without leaving OG.
Here are a
few ideas, each with a specialisation for a purpose; there are many more. The inverse-balloon takes about 30 seconds to draw: stick it in your stencil, choose it whenever you like. Edit/Shapes/MakePointsEditable and you can change or enhance or beautify the shape to your hearts desire, as in the shape of the inner balloon, etc. I've provided them all (apologies) in one
ExampleDocument. Obviously, you need to use only one idea, and use it consistently. I like referencing PDFs stored in one location, rather than moving globs of images around the planet ... not to mention the filesize limit on this site.
2.a I find the inverse-balloon alone (white, with opacity set to say 50%) is enough in itself.
2.b Stick the text in the ballon itself, as in
CAgirl. (Remove the diagonal, that is there for the yellow call-out.) Change the text colour to eg. pink, and it will stand out like, well, pink-on-grey.
2.c Or you can set the opacity of the original page (facebook or whatever) to 40 or 50%,
in the first place, and then your annotations stand out on top. That's what I've done with the sunset photo.
2.d Use a balloon bright enough, and offset enough, that it will stand out by itself. As in
Da Whole Page.
2.e Chris Heath has provided some beautiful glass buttons, as a stencil. I use them
alone, when I want the diagram to look really elegant, with footnotes at the bottom. Anything 3D on a 2D background stands out. I've used the glass buttons for numbering the items in the example, with the explanatory text in this post.
Use whatever suits you. There is no limit to ideas, really. The examples I have shown here are all very fast and very easy in OG (as is). I am not a graphic designer, if I were, I would give you 20 or 40 ideas, not just four.
Cheers
Derek