Is it a disapproving concept? Does it mean a solution that seeks to fix interrelated problems one by one instead of addressing core causes? I ran a google search but couldn't tell much from the seeming rare contexts.
A patch work typically denotes "temporary fix" (at least as used in software development fraternity).
Sometimes the real reason behind might be known but it would take too much effort or significant change that cann't be undertaken either due to cost reason or other structural concerns. In such cases, if possible one may apply 'patch work' as a partial and temporary solution.
Note: A small code change in software is also called 'patch'. I think that term has it's root from patch-of-cloths.
Yes, it is derogatory. Yes, it implies that it was not a well-coordinated effort, but rather a haphazard conglomeration of small fixes.
This is like SOME patchwork quilts. Some, however, have intricate, symmetric designs. http://www.google.com/search?q=patchwork+quilt+images