You might be able to frig something with a post-processing script - tvheadend can pass the user who created the recording. From the help:
Post-processor command
Command to run after finishing a recording. The command will be run in background and is executed even if a recording is aborted or an error occurred.
Use the %e error formatting string to check for errors, the error string is empty if recording finished successfully.
Support format strings:
Format Description Example value
%f Full path to recording /home/user/Videos/News.mkv
%b Basename of recording News.mkv
%c Channel name BBC world
%C Who created this recording user
%t Program title News
%d Program description News and stories...
%e Error message Aborted by user
%S Start time stamp of recording, UNIX epoch 1224421200
%E Stop time stamp of recording, UNIX epoch 1224426600
(formatting gabled, but you can still follow it).
Now, XBMC would still point to the Recordings directory as defined in tvheadend - but you could perhaps sort by user using this mechanism, into separate subdirectories or similar?