I use, mainly the netinst.iso myself - and for Debian5, debian7 and debian8 it has worked (debian 5 on Power4 - standalone and Power5 - virtual, but need a whole disk), debian6 I got to work - eventually (Power5 only tried), but needed to patch yaboot everytime so I waited for Debian7. Debian7 worked in virtual mode, virtual disks (but not when using fancy disk management). Debian 8 I have only tried as a virtual machine on Power6.
The biggest difference though is bare-metal, or virtualised - setting up the VIOS to support an install in whatever way. Maybe USB could be used as an install source. Sounds simplier with bare metal - an HMC can assign a USB port hard into a partition, but it is less mobile - and I do not know if IVM (integrated virtual manager - in any case VIOS as "HMC").
I have written those instructions on my Power/AIX portal and am willing to rework them to fit into a Debian FAQ format.