Well I've currently got it booked for opening night, so hopefully I will find out.
Looks like the breakfast buffet is on hold for awhile:
A note from Hyatt Regency Tokyo Bay
WE ARE CLOSELY MONITORING THE COVID-19 SITUATION FOLLOWING GUIDANCE FROM THE WHO AND LOCAL AUTHORITIES. PRECAUTIONARY MEASURES IMPLEMENTED INCLUDE INCREASED SANITIZATION OF PUBLIC AREAS,TEMPORARY SUSPENSION OF BUFFET SERVICE, STAFF WEARING MASKS AND GUEST TEMPERATURE CHECKS.