Using 'comfort link" thermostat to lockout a stage.

Have a 2 stage HP (Trane XL20i) controlled by a TCONT900 ThermStat (appears to be a Honeywell VisionPRO) using 3 wire data link.
Would like to be able to lock-out 2stage heat on HP (at times). . With only a datalink there's no Y2 to disconnect.
Was surprised that with all the config code options (in installer setup) , the TCONT900 doesn't appear to provide that particular lockout option
Anyknow if there's a maintenace or diag mode that allows this?
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