Using the high temp of the exhaust gases to preheat the primary air as Kristijan suggests is one of the easiest ways.
If you use EGR, try to feed the exhaust as hot as possible into the gasifier (insulated pipes).
Another use of the hot exhaust gasses was done by the DEUTZ-company. They built a tractor, were the spare wood was stored in big tight bucket in cages. The hot exhaust was led through this buckets and dried the wood. A picture of this tractor can be found in the PEGASUS-book (http://www.driveonwood.com/library/pegasus-gasifier/) on page 29. The long bucket lies on the rear fender. A smart idea in my point of view.
Using a turbine to drive an generator sounds nice. You could also run a thermoelectric element to create electric energy.
Creating HHO sounds nice first, but it is not without reason called “detonating gas” in German. Feeding it into the burning zone of the gasifier might not a good idea, better instead directly into the engine manifold.
But here we are more in the field of a thought experiment again, quite impractically for mobile applications in my point of view. What we discuss here might be of more interest for stationary applications, where space an weight is not a question and you run under more stable load.
My choice for mobile applications: Find a good balance between heat economy/recovery and compact build/weight for the gasifier to get rich gas with reasonable effort. And try as much inner streamlining of all the piping to avoid unnecessary pressure and thus power losses.