The best suggestion in the book is to mount everything on a trailer and drag it along, so you can disconnect it when switching back to gasoline.
When will I ever learn not to say "you can't ..." about anything technical.
I knew about producer gas too -- I used to think it would be cool to have a setup that used firewood for the fuel stock (I saw pictures and diagrams somewhere). Coal or firewood could also be used for steam power or sterling engines. A modern Stanley Steamer would be fun to drive -- although it would also probably be expensive, inefficient, and a lot of bother.
What I meant, of course, is that it's much more practical to convert the chemical energy in coal into electrical energy -- then back to a different type of chemical energy in a battery, or fuel cell. Goal gasification also shows promise -- it's big advantage being that existing distribution infrastructure, and ICE technologies could still be used. The winning technology will be determined by a combination of relative efficiencies, total system costs, and market acceptance.