Oh I wasn't so much looking for a library; rather considering writing a serializer myself that I could use in something like the way I described above. I thought it might make it very fast & ergonomic to create axum routes to serve html fragments to htmx clients. Unfortunately the more I sketch ideas out, the less viable it seems (to cut a long story short - the required 'Builder' ends up so large & complex that I might as well just use templates).
Anyway, I'll have a look at those links. Thanks.