I'm not aware of any FCC opinions, but I can give my own interpretation, which is: a sentence is fine, as long as it keeps to a single thought and doesn't go to Faulknerian or Hawthornean lengths. If it serves the purposes of identifying and ending the transmssion at pretty much the same time, there isn't much room for complaint.
The intent of the reg is that if anyone hears you doing something problematic, they should be able to find out who you are by listening from that point onward to catch your callsign. So be courteous and don't cause problems. Keep your signoff brief, and, if in doubt, keep it pro forma. The cases of "[call] testing" and "[call] monitoring" are standard practice, and certainly no one objects to a CW operator sending "pse k" at the end of a CQ. Something like "[call], wishing everyone a happy new year, clear" is arguably bad form, but unlikely (in my opinion) to draw you a fine or a letter. Maybe a postcard.
I did some searching of violation notices on fcc.gov containing "97.119" or "end of each communication". Many of them, and the majority of the fines, involved operators who deliberately didn't ID at all, usually in together with jamming, playing music, or non-amateur activity. One amateur who accidentally left the radio on and a CW keyer sending an endless string of dits was asked to send an affidavit basically convincing the FCC it wouldn't happen again. A few were dinged for chatting 20 minutes or more without IDing, and one for making "several short transmissions" without ID. I didn't come across any complaints against operators who had too much to say after their final ID.