> I was trying to do this with the command line.

OATEST-02> exiv2 -M'reg ns myNamespace/' -M'set Xmp.ns.MyProperty
metadata' test.jpg
OATEST-02> exiv2 -M'set Iptc.0x00ff.0x1234 metadata' test.jpg
OATEST-02> exiv2 -pa test.jpg
Iptc.0x00ff.0x0034 String 8 metadata
Xmp.ns.MyProperty XmpText 8 metadata

> Maybe I should ask if it is better from a
> data intent standpoint if I should add my unique note data to the Iptc
> or XMP metadata?

That depends on what you're doing this for. If you're looking for
compatibility with other software, XMP is probably the best supported
and standardized way to add such custom tags. For compatibility with a
specific other program, just try it out. If you have thousands of
pictures and performance is an issue, IPTC would be better as
processing IPTC is faster than XMP.
etc

> exiv2 -M"add Iptc.User.Country String Canada" CRW_9003.THM
> -M option 1: Invalid key `Iptc.User.Country'

For IPTC custom keys, both the group ("User") and tag ("Country") must
be hex numbers (the group number is only a one byte number) and they
can not be associated with any group and tag names. See the example above.
XMP groups and tags on the other hand only have a name, no associated
number.
That's just how these metadata types are defined.

Andreas