There is no direct support for this in Lightroom SDK.
There is some potential to access image data via previews or manipulate image data via tif files, but I've yet to see it done via a plugin - maybe you can do it though.
thanks for the answer, but how is it that some third partys manage to develop LR effects?
Its all marketing B.S. - They all just export tifs then do their thing to the tifs (like any stand-alone image editor), and what you end up with back in Lightroom is a tif stacked with the original. Some of them have presets too that allow you to do that with only one hand, but its mostly smoke and mirrors... You could do about the same thing with any non-Lightroom-aware image editor just by configuring it as an external editor (other than the presets that let you do things without explicitly opening the app).
Summary: Lightroom "imaging plugins" are not what they sound like. They are:
0. An installer...
1. Plugins that collect app parameters, and initiate the export of a tif, then open an app which:
2a. You use to do stuff to the tif, or
2b. It does its thing to the tif file then closes.
3. Presets and/or menu items so you can do the above without going through an explicit export step...
ok, nice summary!
So bad this is not (yet?) possible.
Indeed - its something strongly desired by plugin writers and Lightroom users alike, but I'm guessing it won't be here for a few more years at least (I think Lightroom would have to undergo some extensive changes under the hood). - Hope I'm wrong.