Can I use the 1723 Surround or Surround S as a ceiling speaker?
The short answer: Yes!
The longer answer: To mount them in the ceiling, make sure you pick up an additional mounting bracket for each speaker. There is one included with each speaker, but you need two for each speaker to mount them securely to the ceiling. Both the 1723 Surround and Surround S are heavy speakers, so make sure you mount them appropriately. If you're not certain how to best mount them, consulting with a professional is always a good idea!
Other things to consider
If the side-firing TriAxial drivers are facing each other, they should be working in phase with each other. If that is the case, The left and right speakers (indicated on the serial number label on the back) should be switched, same as for the surround back speakers (seen here). If they are facing toward the front and back, they should be mounted according to the regular surround speakers.