No modern MIDI instrument will actually "play" a note with vel=0. When a MIDI instrument sees Vel=0, it translates that to Note Off internally. Check your MIDI Events in Step Edit mode... I don't believe you'll find any events with vel=0. Actually when you think about it, Vel=0 is used to determine the Duration (D) of a note event in the Step Edit. Otherwise notes would play forever with a master keyboard!
One way you might be able to get the Akai to trigger "release samples" when a key is released would involve a computer program, such as pd, listening to your master keyboard MIDI events and transmitting "extra MIDI events" when you release a key. Pd is a free program (the precursor to Cycling 74's MAX/MSP) that can be programmed to manipulate MIDI events in a virtually unlimited fashion. It's a VERY powerful program, but has very little requirements (it will even run in Linux, as well as Mac and PC). You could also use MAX/MSP but it costs $$$, but if you already have it, you're good to go.
You could create a fairly simple Pd/MAX/JMax/etc. program that would do two things...
1. pass all incoming MIDI thru to the output unchanged
2. when a note off is detected, pass an extra "quick" (short duration) MIDI Note On/Note Off event to trigger your "release sample".
One way to make this work efficiently would be to first create a 2-layer Harpsichord Program on the Akai. Layer 1 would be your normal Attack/Sustain samples set to Note On mode (so it will sustain as long as you hold the key). Be sure to assign velocity 2-127 to this layer. Now Layer 2 has just your Release samples, probably set to One Shot mode, and you assign it to velocity=1 (range 1-1), which means it only plays when the incoming velocity is 1. This would be the velocity that pd/Max/JMax creates in Step 2 above, that is sent when you release a key on the keyboard.
Now when you play a key and hold it, the Akai plays the Layer 1 sample... and when you release a key, pd/MAX/JMax tells the Akai to play ANOTHER note, this time with vel=1 which will be your release sample.
The only "problem" with this approach is that you couldn't have variable velocity with your release samples. Layer 2 would have to be set so that it ignores actual velocity (otherwise it would play with a vel=1 and would be inaudible). So you'd probably have to pick a fixed volume for your releases (i'd guess you'd need to make them fairly soft, so they're not too loud when you actually play soft).
To get around this (and have full velocity for both attack and release) you would have to split a Program down the middle, for example assign pads 1-32 to your Attack+Sustain samples, and pads 33-64 for your Release samples. This gives you less than 3 octaves to (literally) play with, and may not be acceptable. Why not just create an Attack Program and a Release Program you're thinking, and have all 64 notes on both? Because unfortunately the Akai MPC1000 cannot play more than one Program at a time. It will only play the Program that is set at the bottom of the MAIN page. Actually I haven't given this part a lot of thought, but I don't believe you can trigger more than one Program at a time from an external MIDI instrument (anyone?)
And then again... if you have a computer, you might as well use Kontakt or some other PC sampler to do all this instead of an Akai (because Kontakt has release velocity support).
