FishEye has a built-in service to generate a patch file for any two changesets or any two revisions of a single file. It is available as a barebones web page that, when called without parameters, produces man-page like instructions. This service is currently not documented on CAC.

Instructions:
https://extranet.atlassian.com/crucible/rdiff/FE

Used to generate a patch file for a diff between 2 revisions of a single file:
https://extranet.atlassian.com/crucible/rdiff/FE/trunk/src/java/com/atlassian/crucible/spi/rpc/EscapingXMLStreamWriter.java?r1=21372&r2=21445