So, I'm a bit against this, both from a mechanics and an RP perspective.
Someone sitting, in any capacity, is at a disadvantage in a fight. From sitting on the floor, in a booth, or even on a bar stool, if someone catches you sitting, you're at a disadvantage as opposed to someone standing with their core engaged, or even leaning against a bar. Even with a barstool, which we think of as pretty open, completely disables a person's ability to dodge out of the way of an attack the moment the feet leave the floor. Being able to hide being in a coded vulnerable state with custom text would need to be addressed.
Also, RP wise sitting places in public venues are actually quite limited sometimes. Hence why people going to a club are willing to pay a premium for a private table just so they know they have a place to sit when inevitably their feet start to hurt. If a place really is packed enough to be considered standing room only, then you have to think about where to sit. There's the bar, but really how many barstools do you see at a club? After that it's the available booths or legit sitting on the floor. I hope you know who is sitting at those booths, else things might get awkward.
Saying this I don't think it would be respecting the ambiance of actual rooms if people were to be able to just imagine chairs into existence for them to sit upon.