I think this might be more what you're after (?):
http://forum.cakewalk.com...-sustain-m2748438.aspx As per the thread Voices - Male v Female are good to test on (as the Female has the loop built into the flac (could be wav) which seems more effective(?)). Also as discussed you can also define the loop to the sfz.
But ... you can also do a lot with the envelope modulation itself. I read of setting one or both of Sustain (Shift+S) and Loop (Shift+L - must be set after, but on a prior node to sustain) on envelope nodes. I 'stumbled' across the menu by clicking in the information bar at the top of the envelope (see in the animated gif below). You can also zoom the timing right out with the mouse wheel (or the menu).
Also .. another tip that may be useful ...
Plogue Sforzando can load the Dimension sfzs. It has a virtual sustain pedal (bottom right of the interface) which will sound continuously until release - even on e.g, the Male voice (non looped sample).
Have a play :)