In the vein of "plugging holes":
ISSUE: How can a client determine which uri schemes the Printer supports
for use in the "document-uri" attribute in the Print-URI and Send-URI
operations?
How about adding a printer description attribute:
document-uri-schemes-supported (1setOf uriScheme)
This attribte specifies the URI schemes that the Printer supports for use in
the "document-uri" attribute in the Print-URI and Send-URI operations.
If the Printer does not support either of these operations, the
"document-uri-schemes-supported" attribute SHALL not be supported.