> Is there a way in SIP to send to a UA some 
> non-SIP messages tunneled within existing 
> SIP messages (assuming that the UA can 
> (and expects to) receive such non-SIP messages)?

Not officially since using sip to tunnel messages
of another protocol is discouraged.

> Should the OPTIONS, INFO, REFER, NOTIFY or INVITE 
> message be used for this purpose?

It really depends on what and when you are
wanting to deliver the information.  The INVITE
should not be used unless a real session is 
trying to be established; and a held call should
not be established just to tunnel messages over the 
sip dialog.  Using a SUBSCRIBE/NOTIFY, REFER, or 
OPTIONS mechanism can be used within or outside 
of a call related sip dialog.  The use of INFO for 
any reason has mostly become discouraged.

Depending on what is being tunneled, the data
can be include within the body, a parameter, or
a new "p-" header.  New content potentially requires
going through the IANA registration process to
avoid naming collisions.

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