if using wood or wood byproducts...
you might go over the toxicity lists : http://www.wood-database.com/wood-articles/wood-allergies-and-toxicity/
if you've adapted or are unaffected by what you're using, you might also keep in mind, your guests might not be...
some of the most common allergies are with the two most popular choices due to their tendency to mask the scent
cedar and pine
the scent of pine alone can triggers breathing and asthma related symptoms
while with cedar, many tend to break out or swell through contact with as little as dust particles.
And of course, if you're using humanure for composting for your crops.. there are some wood that might inhibit plant growth in excess quantities.
Though, I'm told a mixture is usually a little safer all around...
In general, if you're going to have a guest outhouse.. making it a bit more roomier inside tends to be helpful for the uninitiated. And it doesn't hurt to have an 'adjustable' seat or more than one place to sit for your super sized friends (might need some reinforcement), just enough space for great grandma and her walker (so maybe some rails) or little timmy being a risk for falling down the pit toilet (and so a ladder down the hole for those that are willing to swim in the sludge for lost wedding bands or iphones).
And with you frugal builders, it should have a rear view mirror and set of tweezers.
of course, you could just keep some blue bags on hand... certainly might help for the my poo is more holier than thous crowd. Although, I get it, there isn't much use for it in and of itself. one of my friends has attempted but hasn't had much luck with trying to redirect that waste to heat to the edge of her property when snowfall reduces the effectiveness of her fence by a couple feet.