I was also playing with some attributes created on the fly to be
serialized but as standalone attributes can't be serialized that
doesn't work out; I am not sure whether creating an element with
attributes on the fly and then serializing it to use string processing
on the serialization result to get only the right attribute
serialization is too hacky.

<xsl:processing-instruction name="my"
select="serialize(saxon:new-attribute('value', string()), map { 'method'
: 'adaptive' })"/>