are there any bents that can be ridden on gravel or dirt trails

Any bent can most likely be ridden on gravel roads or dirt trails, it is up to the rider and skillset to determine whether if they could be any good off-pavement.

Personally, the bigger the wheels the better they roll over rough terrain. Momentum is key in stability.

Right. The problem is the same as with regular road bikes. The tires are narrow and slip off rocks because they're usually at a pretty high pressure. If you put on wider tires and lower the pressure, like you would on a MTB, you should do fine. In fact, the 26" wheel is the same used on many MTBs.

As mentioned above, pretty much anything will work, as long as it'll take big, wide tires (like 26x1.75 minimum). Unfortunately, that eliminates a lot of otherwise good bikes.

Scott P
Bend, OR

RANS VRex (which will take wide tires no problem)
RANS Force 5 (which most emphatically WILL NOT!)

BikeE FX and BikeE RX work really good on gravel or dirt.

Trikes are a blast on gravel. I like to make mine skate around when I'm on loose stuff.