I wonder if one couldn't somehow rig a filter sock to perch on the tank fill. A fabric filter could be folded/ shoved down the fill hole. As long as there was a solid ring on the end of it with a diameter larger than the fill hole, it could never fall in.
When the sock gets dirty (or isn't needed because fuel quality is not suspect), just pull the sock out by the ring and chuck it.
What about a pump nozzle "condom filter"? Slip (or unroll, even) a condom-shaped filter over the nozzle before putting it in your tank fill hole. Fill 'er up, pull the condom off and throw it away, or even save it in airtight container for a fire-starter later at camp? Or a re-useable version: a 6" length of inner tube with ID to fit snuggly on the nozzle with a cylindrical filter sewn or staked in on one end.
I guess the flow rate of the filter is the big concern.