I've ridden long distances on a 20" wheeled, singlespeed childs bike before although it wasn't a BMX. Same as the others mentioned I had the seat up high and I did manage to mount a rack of sorts and carry panniers.

So my reccomendation would be to trial a long seatpost, use SPD pedals/shoes for safety reasons out on the road as well as efficiency and with a long seatpost you could mount a seatpost rack and carry light panniers. Long distances with a backpack would be murder.

Regards, Anthony