You might need to ask someone from the hotel industry if they have any special meanings to them, but 'open' would imply something that is available to use, as in 'we are open to the public', while 'pipeline' usually tends to imply something in development, such as 'we have a room refurbishment plan in the pipeline for next year.'

"Open rooms" are hotel rooms that are ready to be used by guests. "Pipeline rooms" are those being developed but are not yet open. Something "in the pipeline" means that it is still being worked on, but will be ready at some scheduled time in the future.