Done some more digging.
In April 2012 Mugunthan (a TI employee at the time - possibly still) says that CPSW doesn't support this feature (at least on TI814X). On being pressed further, it seems like the hardware isn't really geared up for it and the software driver doesn't (at the time) support the feature, but in theory it could be possible:
http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/716/t/183933.aspx
In January 2014, Mugunthan submits a patch that appears to add at least partial support:
http://www.spinics.net/lists/linux-omap/msg102618.html
It appears here too, but with an older date:
https://gitorious.org/am335x/linux-am33x/commit/a867d485bc895237d5690ec6ba3e536ce2796200