I'm very happy to hear a stealthRL in progress. I'll definitely give it a try when/if it comes out.
Facing would be easy: One dot of noticeble color on the direction the enemy is facing (only displayed if enemy is visible though). Cone of view is tricky however. Maybe it can have two ranges like in Commandos. Cones can start with 3 dots and increase in width every 2 dot (or whatever). If you don't have 8 facings (cones could be difficult on good ol' dotted RL screen) and settle with 4, cones should be wider. Enemy comes to investige if @ is in outer range, sees @ and raises alarm at closer range. Only inner range is displayed, outer will be double/triple in range. Colored tiles is definitely the way to go.
The levels will have to be sparsely populated if the game is to be playable. Unless you can casually kill enemies, in which case it'll be an ordinary RL. The enemies difficulty should increase their awareness, not toughness (which would be extreme anyway). A weak enemy rarely changes his facing, has a short and narrow cone of sight, can ignore @ at outer range, ignore some sounds, and has hearing/seeing penalty for whatever stealth checks. A strong enemy routinely looks all around, moves around possible erratically, has a very long and wide cone of sight, hears very good, always investigates all sounds and @ at outer range and has bonuses on checks.
But either one would one(or two)-shot the @ should he be noticed.
Hope you make it.