I'm not certain which are possible to disable on a per-actor basis but an ideal list would be:
Sprite rescaling
Texture filtering
Anisotropic filtering
Dynamic lighting
Sprite smoothing
Sector colour (that's if it even affects sprites)
Multisampling ~Probably doubtful
Here's a picture by Apeirogon showing the two Fill Spectres in development:
[imgur]https://i.imgur.com/wxA18fx[/imgur]
The one on the left shows the outline created by texture filtering and the right one has an "infinite" sized sprite for chroma keying which avoids the previous' artifacts. Both always result in compromise though because the left one imposes many graphical restrictions on the end user and the right one loses compatibility with other colourful mods.
The flag I'm requesting would come with the potential for abuse and I have a proposal for that too: Force +bright when it's in use, as that's required for chroma keying anyway

Thanks for reading!