Lots of interesting stuff on the forum lately about fuel cells, engine generators, silent IC engines, linear generators, etc.
The back of my mind has always bounced back to the linear generator concept as seeming to make sense. The "fuel" issues seen in fuel cell discussions lead to a why not the ability to use any heat generating source or fuel, which lead all the way back to the venerable Stirling heat engines.
Expanding the above, it then becomes a question of if you can take the rotating mechanisms out of a Stirling engine and make it a linear generator, and run it on anything from propane, gasoline, diesel, van waste heat, etc.
I have cast around in the past without a lot of decent information found, but I just ran across this Website page.
As it turns out, the linear generator Stirling powered device has already been produced by some companies that are still in business.
The link above shows one and has lots of links to other websites of manufacturers that I haven't yet had a chance to look at closely or contact them.
There obviously have to be some reasons the principal hasn't been taken to more market yet, but it sure seems to fit the requirements extremely well. Simple, scaleable, any fuel, likely no temp restrictions, etc.
Los of reading to do, I think.
This is a 2011 article, so things could be a bit different by now.