Would a hard tail 29er be a good starter bike for road and light trails? I can't really figure out what kind of bike I should be getting as a "loaner" bike. I have plenty of friends who want to go for a ride but have no bike, and ever since my last loaner got "borrowed" 2 years ago by a friend after the ride, I've been riding solo :(

I have a road bike and a full suspension mountain bike. I do light to medium trails half the time I'm riding, but I cover double those miles on when I'm on the road. 

I want a bike that can be a catch-all for friends to join me. Previously I had a hard tail Specialized of some sort with not knobby tires on it that I left between 50-80 PSI. I also never had had a 29er before, and if I'm going to get another bike, I may as well have use for it too. My full suspension is overkill for the local rail trail.

Chances are though, I'll end up on a few good 20-50 mile road bikes with these friends, and I don't want to make them hate the sport due to having the totally wrong bike. 

Hoping to get some feedback on how good or bad of an idea this may be! I'm eyeing a used Specialized Fate that's priced relatively well on Craigslist. (But there's a hundred other 29er hard tails out there!)
Short answer yes, a hard tail 29 would be a good fit for what you have described. And it sounds like you already have a fun FS bike and a road bike so it only makes sense. The equation N + 1 comes to mind, where N is the number of bikes you currently own.

29ers roll like nobody's business, they don't accelerate like a smaller tire but the roll over is awesome. For longer 20-50 mile rides a hard tail would not hold you back either especially if your using a fork with speed lock out and smooth tires to reduce spent energy. It won't keep up like a road bike but it won't be completely unbearable either. But a road bike taken on a trail could get interesting quick for an amateur rider.

Sounds to me like you know what you want and have narrowed it down pretty good on your own. I say go for it though.