I use the AUX button for my DVD. I too thought it was strange that a surround receiver remote did not have a dedicated DVD button especially since the source buttons at the bottom do have DVD.

I have the exact setup as you. I listen to Cable using the TV volumn and DVD's with the outlaw. It can be done but not perfectly. The only way I have found to modify the volumn lock is to use advanced codes to re-assign the volumn up/volumn down buttons to the advanced codes for the TV. However this means that this can only be done when the remote is in TV mode not when it is in CBL mode. This bugs me a little since I use the remote to control the TV though a cable box so I have the remote in CBL mode all the time for channel surfing and then I hit the volumn and it does nothing. I have to remember to switch to TV first. So again it's not perfect but at least I don't need two remotes.

Here is how I set mine up.
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1) Use the volumn lock to assign the volumn to the receiver (I think you did this already)
2) Assign the DVD player to the AUX button. Instructions on Page 33 as Gonk pointed out.
3) Assign advanced codes for volumn up and volumn down for TV mode.
To program an advanced code you can follow these instructions taken from this manual .
a) PRESS the device, (TV), button - RELEASE
b) PRESS and HOLD the SETUP button until the red LED flashes twice, then RELEASE.
c) PRESS 9 - 9 - 4.
d) PRESS the SETUP button and then the three digit advanced code. (Here you need to find your TV's advanced codes for Volumn-UP and Volumn Down, for example my Sony's codes are 076 and 080 use this page to find yours. Find the CODE you used to control the TV on the page and then click the code to see a list of advanced codes.)
e) PRESS the button you wish to use for that function - Two Blinks. (In your case the Volumn Up or the Volumn Down)

If you mess things up don't worry you can return the button to it's original setting by doing the following to clear out your override (also from the manual above).
a) PRESS the device, (TV, VCR, CABLE, etc.), button - RELEASE
b) PRESS and HOLD the SETUP button until the red LED flashes twice, then RELEASE.
c) PRESS 9 - 9 - 4
d) PRESS the button you want to clear twice. - Two blinks.

WARNING DON'T USE CODE 981 to return to factor defaults as this erases the outlaw receiver codes put in by outlaw.