Simon,
You may be able to manually configure the mask register using a CCS memory window. The location of the register is documented in appnote SPRA820, section 2.1:
http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=spra820
You'll need to do a little bit of observation to determine if Analysis unit #0 or #1 is used for the data (vs. the address) of the watchpoint. Also, you may have to modify the CCS memory map to allow you access to these locations (this is on the tools menu I believe in CCSv5, or you could modify the .gel file that goes with your target configuration to add the addresses for the analysis unit registers). I have not manually poked the analysis registers in combination with CCS before. The appnote does show how to write C28x resident code that will configure the analysis units, but you want a combination of using CCS and manually configuring the data mask register only. You can give it a try. I suppose if it doesn't work and you really need the watchpoint, you can use the technique in the appnote to add the code to your project and just configure the watchpoint completely there.
Good Luck,
David