According to the UUID specs at http://www.ietf.org/rfc/rfc4122.txt
I quote (this is under the v4 section):
Conformance with URN Syntax: The string representation of a UUID is fully compatible with the URN syntax. When converting from a bit-oriented, in-memory representation of a UUID into a URN, care must be taken to strictly adhere to the byte order issues mentioned in the string representation section.
Validation mechanism: Apart from determining whether the timestamp portion of the UUID is in the future and therefore not yet assignable, there is no mechanism for determining whether a UUID is 'valid'.
** I presume that if no validation is possible, no software should complain about your manually generated UUID, as long as the timestamp is future