I'm with 763 jock
I support 763 jock's perspcetive.
If you go to Boeing's Volume I, you will see that the APU-to-Pack procedure is only valid for the 757 due to its different Pneumatic System. Any use of the APU-to-Pack procedure for the 767 would be contrary to Boeing procedure.
Think of it this way....the reason you are doing a Pack-off takeoff is because you are performance limited. Now try running the APU during the takeoff, and you have a few problems for the 767. First, like 763 jock says, there is not enough pneumatic air to power both the pack and the ADP. Boeing tells us this. If you lose an engine, then you now have a very slow or even no gear retraction in a performance limited scenario. Not good. In reality, it is actualy worse than what your performance says because you have the APU door open. On the 757 that is a 500 pound hit on performance. I don't recall what it is for the 767.
I fly both the GE and P&W version of the 767. They are similar with this particular issue.
Also, as 763 jock states, the Performance hit for an ADP inop is huge for the 767...on the order of 40,000 pounds.
I'm with 763 jock. Since it is not an approved procedure in the Boeing manuals, and it also is clear to me why it is not approved, I will never do it in a 767.