I spent about 20 minutes in the 1-hour "mock contest" last night, an event set up to test the feasibility of using FT8 to send and receive contest exchanges.
The event used the ARRL RTTY Roundup exchange which, for NA participants, was "RST State/Province".
Upon selecting the RTTY Roundup in WSJT's menu the user is prompted to enter their exchange info. I inputted an arbitrary signal strength and entered "599 TX" into the field. I should not have entered the 599 or any signal report - I learned during the contest that the software will generate a signal report based on the received station's dB level.
While my Message Field looked erroneous (with two signal reports inserted), the actual transmissions sent were correctly.
In addition to correcting my error, the software worked without any hiccups and it was nice to see the state/province of the received station being displayed.
Although I have never participated in a RTTY contest and have no interest in doing do, this method of operating a RTTY-like mode does appeal and I'm looking forward to operating FT8 in an upcoming compatible contest.