Why not just ride with a mild snaffle without contact? I initially just trail rode my Arab (and still do sometimes) with the same snaffle we do arena work or foxhunt in (http://www.chicksaddlery.com/page/CDS/PROD/3030/KS4184).
Or even bitless? I do trails and endurance rides with my Arab in this: http://www.runningbear.com/catpages/Smart-Bridle-Photos-SP.html He loves it. Its essentially a glorified halter with rings on the side of the noseband to make it a sidepull. The little rings halfway up the cheek pieces allow you to clip a bit or hackamore to it if/when you need to. I ride on a rather loose rein and just kind of neck rein (he can get his head most of the way to the ground), but mostly use seat/leg for steering.
My Arab wears this: http://www.chicksaddlery.com/page/CDS/PROD/1040/238315 when we play around western. He seems to like it.
Mullen mouthpieces are generally quite mild. Some horses like solid, ported mouthpieces. Shorter shanks will typically be milder than longer ones. Swept back “grazing bit” shanks are also often milder because they get more time to react before the rein actually pulls on the shank.
I don’t think a Kimberwicke is a lot of bit if you’re not riding with contact? Although, I don’t really like them as a general rule. I think they’re a weird compilation of a snaffle and a curb that doesn’t really do either job well. If I need something more than a plain snaffle for a come to Jesus meeting, I go to a mullen mouth Pelham with two reins. I’ll ride mostly off the snaffle, but will use that curb rein to enforce things when necessary.