applying effects in plugin

I want to do something like the mask/erase effects for construct, but via a plugin. the plugin will be drawing textured quads, and I want to be able to take, say, 100 quads, and have that applied as a mask to the next 100 quads. if the answer of how to do this is too involved, it would still be useful to me to know if it's possible at all, so I don't go on a wild goose chase.

If you can't do it by the ordinary IRenderer methods, the only thing I can think of to do is then call IRenderer::GetDevice() which returns a IDirect3DDevice9* (but returns a void* in the SDK so you don't need the headers). Then you'll need the full August 2008 DirectX SDK (IIRC). Using the device directly is tricky since the runtime expects to have full control over it - make sure you restore any settings you change.