As a workaround for the moment:
If you use my Ultraschall-API, you can use this code to get screenshots.
But: they are the size of the video-window. So, the bigger the video window, the bigger the screenshot.
And the video-window must be fully visible, not behind other windows or they will be captured as well. And it works only on Windows yet.
Code:
dofile(reaper.GetResourcePath().."/UserPlugins/ultraschall_api.lua")
hwnd = ultraschall.GetVideoHWND()
retval, left, top, right, bottom = reaper.JS_Window_GetClientRect(hwnd)
ultraschall.CaptureScreenAreaAsPNG("c:\\tempvideo2.png", left, top, right-left, bottom-top)
The idea with capturing via gmem could work. I look into it at some point.