The default limit of how long a filename can be on EXT3 is 255 characters. I have a unique requirement where I need much longer filenames (apparently because mod_rewrite of apache uses filenames for storing). Is there any setting I can tweak to increase this limit from 255 characters? (Or if I can change mod_rewrite setting not to use files)
This is a fixed limit in ext2/3/4, so unless you can change filesystem or the length requirement you are stuck.
Reiser is the only common filesystem that springs to mind as supporting longer filenames than that (I know someone who chose it over ext2/3 for that reason, though I forget why he needed to support very long filenames).
The length limit is per name, so you can have paths longer than 255 characters as long as individual names along the path are not over-sized. This maybe enough to deal with the situation if you can update your rules to output paths of the form "/store/200-character-dir-name/200-character-dir-name/200-character-file-name" rather than "/store/600-character-file-name". Though requiring very long names with or without splitting it into paths for shorter names seems odd enough to imply there might be an overall design problem to be addressed (I say might as there could be good reason for the requirement!).
Be aware that even if you change filesystem or use the path approach to split the long names, some tools may not like the long names/paths (some archive/compression file formats won't support either, for example).