Unfortunately this has not solved the problem. It has filled out the horizontal dimension (no more black bars left and right) but it is still displayed at the wrong aspect ratio.
From
this pair of screenshots you can see the problem - the image above is the view before playing (correct thumbnail that TWG provides extracted from the flv) while the image below is taken during playback, and quite clearly is the wrong aspect ratio (with the top and bottom cut off).
I have tested the flv with VLC player, Realplayer, Adobe Media Player, and QuickTime Player, and ALL play the file correctly with the correct aspect ratio. Plus I have tried multiple encodings, at different settings, using ffmpegX, with no difference - the problem with the TWG gallery player remains.
Update: The only thing I can think of that is "strange" about the FLVs I am having this issue with is that they were all generated from the same source MOV file by using QuickTIme Pro to cut the MOV file into chunks, before converting to FLV. Perhaps that has something to do with some players being unable to read the length (Flash Player and Real Player also have this issue, though QuickTime Player and VLC Player do not) though it seems to be a separate issue from the aspect ratio problem.