Your editor mod sounds great.
I'm making my own editor mod, but very different to yours. Mine is a Style Changer mod that changes the style of existing elements, walls, and elevators, rather than adding new ones. You just run my StyleChanger.exe to choose a style from: 1950s, 1960s, 1970s, 1980s, Portal 1, Destroyed, Behind The Scenes, Evil Wheatley, or Clean. Then any map you preview or compile will be in that style, complete with textures, entrance and exit elevators, testing elements, ant-lines, voices, etc.
It's nearly finished.
My mod also includes replaced vbsp.exe and vrad.exe that add the following features, which can be activated by adding codes to comments in the editoritems.txt file: removing the entity limit, automatically packing certain files, fixing the bug with messages not being sent to barrier hazard models, replacing wall/floor/ceiling textures, changing signs and antlines, changing barrier hazard and barrier extents textures, changing the skybox, changing goo into bottomless pits, compiling previews with faster lighting, and including the entrance and exit in the preview if you press Ctrl+F9. And of course it can select which editoritems.txt file to use.
I don't know if any of those features are useful to you, or if there's some other features you want added that require automatically changing the .vmf file before it's compiled.
I can also get silver Reflection Gel working and looking right, and have purple Adhesion Gel, both in the same PeTI map, and with all the other gels still working. The Adhesion Gel doesn't do anything different from the Reflection Gel though, just looking different so you know to use it on cubes to stop them sliding off Propulsion Gel. It requires some materials and particle systems to be automatically packed though.
I don't know if you want to collaborate together or help me make it so my style changer is compatible with your mod. Or if you want me to help finish your mod for you, or whatever.