For the extending graphic sets with new objects, I've mentioned what should be suggested before in
this post. I'd like to make a few changes in my suggestions for the objects for the official styles. As you've already done the LPII styles and Xmas, they won't be listed here.
Dirt: 1 remaining slot - updraft? (you may have better ideas for this one, though)
Fire: 1 remaining slot - radiation
Marble: 2 remaining slots - splitter, slow freeze (I'm thinking of this partly because more styles need slow freeze objects, and partly because this was inspired by the statues that you see in the PSP version of the Marble style)
Pillar: 1 remaining slot - locked exit and button? (but this means you will have to edit the existing exit).
Crystal: 1 remaining slot - slow freeze (this one should reuse the radiation graphic from the Desert and Sky styles, but it should be blue)
Brick: 2 remaining slots - splitter, locked exit and button?
Snow: 2 remaining slots - updraft, slow freeze (same as suggested in the Crystal style, but in a white color)
Bubble: 1 remaining slot - radiation
Sega: 8 remaining slots - locked exit and button, teleporters, one-way fields, splitter, radiation/slow freeze?
I recommend starting with the Sega style first.
For a proper readme, I'm not sure if you should still do one since all the information about NeoLemmix that people need is right in the website.
For the in-game configuration menu, do you mean a type of menu that has all the settings from the INI file, but with a more user-friendly interface, which can be accessed from the main screen of the player? If so, I strongly like this idea. It will also work very well in the traditional players.
What is the highlight lemming?