The question isn't if but how much the image quality will be degraded by an extender. I would hope it to be less than a 2x digital zoom... Keep in mind though that extenders steal about a full stop of light and limit the usable imager size.
Also found this one in the list of the usual suspects: Tamron 13VG20100AS
There is this lesser known manufacturer who has a broad lineup of fix focal length lenses for CCTV and machine vision: VS Technology, they offer lenses with up to 100m focal length in various qualitiy grades (SD - Multi MP).
Got another trick up my sleeve, although this one borders on cheating: You can use a camera with a 1/4" Full HD imager and crop it to 1.3 MP. The pixels and accordingly the field of view are about 1.7 times smaller, making it look like you are using a 1.7 times longer focal length. This will give you a smaller field of view, but not necessarily more resolution.
SLR lenses usually state their "true" focal length and thus behave like a CCTV lens with some crazy mount. However, if the lens is made for smaller sensors (eg. Canon EF-S, Nikon DX) they sometimes state a "Full-Frame equivalent" focal length. This one already takes the crop factor from above into acount, so divide by the crop factor to get to the true focal length.