I did some digging and some testing the other week, and it appears that no data concerning spectators is sent from the server to non-spectator clients. This means it is impossible to have Omniscience make spectators visible to you. You're out of luck.
During the investigation, however, I noticed a potentially useful feature and added it. Normally only accessible when in Spectator mode, there is a hotkey that highlights all players in the field of view. I have modified it to allow players in any gamemode to use it, given they are not disallowed the permission. Check your key bindings in your settings, this feature may be unbound by default.
Server owners/administrators: this mod supports ClientPermissions.