As a Lux main I've played Support Lux before. It's fun and alright; even though it isn't Ranked material it still has its moments in Normals.
For masteries and runes, consider exchanging any defensive stuff for AP stuff since Lux is always in the back abusing her range, but make sure you still have the utility neccessary on Supports.
I start with her Snare (Q), and I max Prismatic Barrier (W) before maxing Lucent Singularity (E).
Like any Support I get Philosopher's Stone, but the next GP10 item I would get would be Kage's Lucky Pick instead of Heart of Gold; survivablilty is nice but when playing Lux you zone yourself safely away from enemies, and she can make more use of Morrello's Evil Tome than Randuin's Omen. Getting Aura items like Aegis isn't too bad, but consider the distance you would have between yourself and your Solo Top + Jungler in teamfights. Instead, consider Aura Items that would benefit those in the back along with you, such as Soul Shroud for cooldowns + mana regen, Will of the Ancients for your AP Carry, or Zeke's Herald for your AD Carry (plus the cooldown will help you). Overall you will build more AP items than supportive items to increase the strength of your shielding and your ability to poke before the rest of your team comes in and finishes off enemies.
As for Lux's Ultimate, yes, it really is more suited for actual AP Mid Lux than Support Lux. You can try poking the enemy AD Carry with your Light Binding or Lucent Singularity and then Ult him/her so they'll be crippled during a teamfight. If you're worried about kill stealing with it, consider using it as a mini-Clairvoyance to see in areas farther than your Lucent Singularity can reach since it has a low cooldown anyway along with your cooldown reduction items.