Does the XML signature standard support Ed25519? https://www.w3.org/2008/xmlsec/Drafts/xmldsig-core-20/#sec-ECKeyValue talks about ECDSA but even the explicit curve parameter stuff won't work with Ed25519 as Ed25519 has a completely different equation with different equations for point doubling / addition, etc.

Does the XML signature standard support Ed25519?
As of 2018-03-12, no.

(Whatever is the latest version may change.)

The XML signature standard looks like a committee of bureaucrats got in a room with a 1990s-era textbook on cryptography and invented a way to shoehorn it into XML syntax by stuffing it into a blender with the X.509 standard and a collection of O'Reilly XML books.  This is not a standard for using—this is a fine standard for laying down and avoiding.