Johan,
Yes, 1PCC supports re-Invite without SDP.
From your post it's unclear who is establishing the original call, and how RFC2833 mode is negotiated. Does your app sets RFC2833 DTMF xfer mode, and if so, what payload is used for it (by default, it's 101)?
Please copy here a part of RTF log with SIP messages and HMP stack internals which includes the second re-Invite and goes up to TASKFAIL