To be more specific, I delved into Defold’s sources.
So, as far as render target formats are concerned, this is the easiest thing to do. We need to prescribe these formats in several places. And everything will work by itself.
As for MRT, things are more complicated. We need to provide options other than GL_COLOR_ATTACHMENT0 in OpenGLNewRenderTarget() (see defold-dev\engine\graphics\src\opengl\graphics_opengl.cpp)
So I have no questions about the complexity of MRT implementation.