If I am a contract that’s composing together multiple SVGs, I’ll probably know the position and z-index of where I want to put my child SVGs. What scenarios do you envision where the child SVG knows what z-index to render at, but the composer does not?
Have you considered using the link tag instead of concatenating SVGs on-chain? Would have to more formally describe the URI format, but as a really rough example:
<svg>
<image width="10" height="10" href="eth://0x1234...abcd/image?u256=34"></image>
</svg>