Bernd Schemmer has an interesting post about using fiocompress for file-system level compression of individual files on UFS file systems...
However, it might be useful to mark files as compressed after compressing the file, such as when you forgot the "-m" option when compressing a large file.
The fiocompress utility does this by calling the _FIO_COMPRESSED ioctl on the file.
There seems to be no way to unmark a file that is marked as compressed. (The ioctl sets a cflag on the file called ICOMPRESS, but no operation to clear the cflag seems to exist)
I stripped down fiocompress to a minimal tool to just mark a file as compressed. It is important to ensure that it is a valid file (outfile in the example) generated by "fiocompress -c infile outfile" before running this command. Bad things may happen if this is not the case.
To mark output as compressed, compile the code (compile.sh should do that for you) and run "./markcompressed -m /path/to/outfile" if you are running it from the directory where it was compiled.
Source code can be downloaded from here. (A quick ugly hack, based on fiocompress from OpenSolaris, with some likely bugs)
As always, you should ensure that you have the latest recommended patchset installed.