Looks like you can embed SVGs in markdown, and they render (tested in Tesseract and Lemmy-UI). So if you link them externally, support is technically there.
Lemmy Logo SVG
EmojiMart also supports SVG for custom emojis. So I guess if you host SVG custom emojis from a static folder, they should work as-is, though you'd need to add them to the database manually (which is a Lemmy UI limitation since it only presents an upload field rather than a URL field).
But yeah, pict-rs seems to be the roadblock for "native" support.
Nice. I figure all the web-based ones should unless they've got a static list of image types that doesn't include svg. The installed apps could be hit or miss depending on how they're implemented.