Oh gosh.. I 'assumed' that so long as I was connecting to an afp share, OS X would index the shared drive. Hmm.. so the netatalk plugin in OMV3 doesn't support indexing. I tried to activate spotlight indexing in OS X, however, I get the message 'indexing is disabled' irrespective of the command I use. I'm not sure if it is a permissions error or just not possible with an AFP share. Thanks for the info.
NAS OMV 3.0.59 Rig: DFI Lanparty nForce4 SLI-DR | Opteron 165 Dual Core (no overclock) | 4GB Corsair Non-ECC DDR400 RAM | OMV3: 1x100GB IDE | Data: 2x 250GB WD 7200rpm, 2x 3TB WD Red, 1x 4TB Samsung 5400rpm