I remember this guy wrote a trip report and detailed a stay there. Scroll down to post #25 for the Hyatt. He doesn't say if he took a girl there, however. If this Hyatt is like the Erawan Grand Hyatt it will be LB friendly but not exactly welcoming. At the Erawan GH, they made it pretty clear they frowned on LBs being in the front lobby or around the reception desk. They make girls go downstairs to a security desk to hand over their IDs and register themselves. One girl I took there was very reluctant to visit any of their restaurants or bars because of that experience. That said, the staff treated her respectfully and they never said anything to me about having ladyboys visit my room. It was LB friendly, but without a smiley face.
Per your own plan, I definitely would contact them in advance and inquire about their guest policies and specifically mention the guest would be a ladyboy. Chances are, with the hospitality industry currently in economic ruin, they won't be too picky about who stays there these days LOL.