Even if, in an idle descent, there is not enough bleed air from the LP bleed, there is always the HP bleed- that is, pretty much, why it is there. Even if HP bleed were not enough, when the A/I valves are commanded open the EECs command "approach idle" and a good bit more bleed air is available. So no problem with lack of bleed air for engine A/I in a descent, I think.
Cribble
When I used to fly 757's (RB211) I was amazed at the number of times that I had to fault the failure of the HP valve to open on descent. I had the reputation with the engineers for snagging the valves more than the rest of the pilot force combined. On the 757 there was no eicas message, is there one on the 777?