This is a feature request:
As a followup of this discussion it might be possible to add a simple XMP write support using the provided infrastructure (that is by passing image.encoderinfo["extra"]). This approach doesn't account for large XMP Data (>64Kb), but would be an improvement.

For an discussion on XMP, especially large XMP fields, see #5076, there are also sample images linked.

To keep the interface simple, it should only accept str in the first iteration, later on some XML structure, requirements for a dict interface would need full support for the XML data model like XML namespace support and lists of elements with the same name.

See also #8069