While positions like these are not common, there are several fields out there that require "field" engineers that I can think of:
Power - For seven years I fielded calls for the Power Industry where 60% of my time was spent on the road or in the air traveling to remote locales around the world to fix the problems the "Homer Simpsons" of the power industry had created. Without internet I used just my know-how of various hardware types, operating systems such as AIX, Solaris & Windows and troubleshooting experience to solve problems. It was fun to travel and a daily challenge to solve what ever issue it might be, but I ultimately gave it up to have a family and be closer to home. The only thing that really sucked however is the remaining 30% of time I had in an office was usually spent in front of a desk writing ANSI, ISO, NEMA and OSHA compliant documentation about my journey's.
Networking Specialist - These people design, install, maintain and troubleshoot computer networks for all whom will employ them to do such. Some companies specialize in contracting guys with CCIE's etc out to companies who do not want to pay to have one full time. They generally travel on short notice and are prone to 60% or greater travel time.
Deployment Specialist - These people are usually certified in some specific product within the company they work for and make a job out of traveling around to "deploy" said product. Everyone from A to Z in Software and OEM Hardware employs these people to do the dirty work of installing and troubleshooting a product on a customer's site after it has been sold. Expect lots of long hours and a lot of travel to go along with these kinds of jobs.
Sales Engineer - Otherwise known as Systems Integrators in some companies, these people help potential (pre-sales) customers understand, compare, and contrast the solutions that are available for buying from the company they are employed for. Companies such as NetApp, EMC, Dell, HP and others use SE's to accompany sales guys to meetings about a potential sale. These people are generally hardware techs who moved their way up in the ranks from within the company or moved from another company doing something similar. As such, it would be best to start as a deployment engineer or similar first if this sounds interesting.
Technical Trainer - Just about every Tech company employs these guys to travel and host various classes, lectures and seminars. It's not overly "brainy" work, but the job does travel... A LOT.
While I am sure there are more, this was an "off the hip" list that I could come up with. Perhaps others can add to it. Good luck in your ventures... It will not be easy and there is no avoiding at least some "office based desk work".