I, personally, can see no reason why you shouldn't be able to do it (once it has been approved by Boeing etc, of course). We used to do a similar thing on the 737 (3/4/5) but I appreciate the differences between the aircraft types. 763 jock has some very good points but the centre hydraulic system's primary source of power is from 2 electric pumps, with the air-driven demand pumps coming from the centre manifold (supplied, in this case, by the APU). The demand pumps are, of course, there to help out the struggling electric pumps during periods of high hyd press demand and I have absolutely no doubt whatsoever that using APU air to power both packs and the hyd demand press pumps will cause wheezy performance of the hyd demand pumps and lead to slow gear retraction and, possibly, flap retraction. At the end of the, however, the systems will still all work and the THEORY works.
A possible solution to this could be to isolate a pack, thereby leaving more APU air to help out the hyd demand pumps. One pack is good to 35000', so you could reconfigure to normal operations before that becomes a problem.
In answer to Plain Plane's question, the APU is supposedly okay supplying air to the packs up to 20000'.