You're welcome. From my experience both 2.5 and 2.6 look in /usr/local/lib and /usr/local for libx264 and use the most recent one. Can you explain what you mean by "In the 2.5 builds it seems that the x264 version required is hard coded in to the executable?"
First I am working on windows, not linux, at least in this particular discussion. 2.6 for linux(Debian in my case) is something I may look in to more at a later date..as a matter of fact, I will most likely look in to that, just one thing at a time.
Anyways, windows builds look in program files or program files x86(in my case the prior since I use the x64 builds).
As for clearing up my statement, I wish you would have asked this just a few min ago. :p I had the perfect explanation open as described by Lord Mulder but I can not find it now but I am trying to find a newer version libx264 so I can throw it in here if I find it again as a just incase. However, I will try to explain it better. When dealing with avidemux 2.5 in windows, the executable is compiled in a manner it will look in the C:\Program Files\Avidemux folder rather then /usr/* and it will only look for 1 specific version of libx264. For example, on 2.5 it looks for libx264-120.dll so if you delete it and replace it with a newer version such as libx264-124.dll. If you do this it is going to give you an error pop up on launch along the lines of libx264-120.dll is missing. However, in my digging it seems some builds are set up in a manner you describe where it will simply scan a dir and pick up on any libx264-*.dll. If I can find a newer version of libx264, I was going to pop it in and see Avidemux will pick it up. Hope that helps.
Also do you know where to find later binaries of libx264 then libx264-124.dll? My googleing is not turning up much in the way of binaries, and Lord Mulder seems to have stopped providing updated builds.