Reducing draw-calls with shared textures

[j-h-a]

I'm still learning graphics programming, so if something doesn't make sense or suggests ignorance of some obvious thing, assume I don't know what I'm talking about. :)

Is it possible, or could it be possible to rasterize all glyphs into the same RGBA image buffer and have a single texture? Full-colour emojis would use all channels, whereas mask glyphs would write into only one channel. Mask glyphs could then overlap each other in the image so they still only take up one byte per pixel. The glyph info (quad data) would contain which channel as well as the texture coords, etc. - would this allow a single shader with a single texture to draw it all in one batch? SDF would probably require a different shader but could potentially use a similar approach and one or two channels based on precision required?