According to the OPC Spec, part 6.2.2

the subjectAltName shall be either a valid URL (see RFC 3986) or a valid URN (see RFC 8141).

So if we want to use an URN here we need to follow RFC 8141,
but we neither use the syntax nor have a valid NID:

According to RFC 8141 an URN has basically the following syntax:

assigned-name = "urn" ":" NID ":" NSS

Where NID is a "Namespace identifier" that has to
be assigned by the IANA.

Furthermore the NID part may for instance not contain a "/" character,
that is only allowed in the NSS part.

There are other issues with the RFC – specifically the addition
of the “r-component”, “q-component’’ and “f-components”
and equality rules that exclude these parts of the URN.