Nice work! That looks great.
If it's ready for general use, please post it to the Viewer & VFS Plugins area.
Would it be possible to make it not require copying SciLexer.dll to system32? Is there a reason it can't be loaded from the same directory as the plugin DLL? (If it's loaded via LoadLibrary, you can give the full path to the DLL; it doesn't have to be in the system path.) That will make it possible for it to work with portable installs, and also avoid clashes with other things that want a different version of SciLexer.dll in System32. (And System32 should only contain parts of Windows itself really.)
Compiling for a static C++ runtime is also nicer for people using portable copies of Opus, since they cannot depend on the CRT being installed.
Supporting portable/USB copies also needs one more thing, which is a USB-safe manifest in the resources, which tells Opus the plugin is safe. (This should only be set if the plugin doesn't modify the registry or anything else outside of the Opus config folder to store its configuration etc.)
Please shout if you need any information or help!