I did not see such a setting and i think it slightly defeats the purpose of using an “editor”.
However, I think it would not be too difficult to add a tool that passes the currently open file to a viwer or even write such a plugin.
I am also curious under what circumstances such a capability is really useful?
Opening a file in the Read-Only mode insure that you will not change it occasionally. In this mode the Editor works as a Viewer while preserving all highlights, tags and other settings.
A possibility to pass a Read-Only flag/switch/parameter from a command line is also might be very useful since it allows opening the files in the Viewer(Read-Only) mode automatically by just passing a file name to the script.