I do actually use the laptop as a second screen in this way most of the time. However, it's amazing how many hotels lack any table deep enough to place a 15" laptop on a stand with a keyboard in front of it.
Wow, that's obnoxious. You may be able to call down and ask for a folding table of some kind. But yeah, you shouldn't have to.
Fwiw, with my 15", unfolding flat and standing vertical, with keyboard etc in front, gave a plausible screen position. So I just kludged vertical-ish stands (and fretted over an eventual tip and crash). I considered hanging a portable monitor off the side (brick of a thinkpad, accustomed to gaff taped extensions), but didn't get to it.