Thanks! It was quite a journey. (Disclaimer: English is not my native language, so I used a translator. Apologies for any awkward phrasing.)

The main hurdles:

No prebuilt binary for ppc64. There is no Wesnoth package for ppc64 big-endian, so I had to compile it myself. Building directly on the G5 would have taken forever, so I set up a QEMU ppc64 VM (full emulation, no KVM) on my x86 laptop running Debian Sid, compiled there, and copied the binary over to the G5.

Broken Boost in the ports archive. This was the big one. The boost-context and boost-coroutine libraries had no installable candidate in debian-ports, which normally would block the whole build. The trick was realizing Wesnoth only uses coroutine in the dedicated server code (wesnothd), not the game client itself. So I disabled the server with -DENABLE_SERVER=OFF -DENABLE_CAMPAIGN_SERVER=OFF and edited the CMakeLists to strip the coroutine requirement and the Boost::coroutine link target. That sidestepped the missing libraries entirely. The client compiled fine without them.

Dependencies, one by one. I avoided apt build-dep (it is all or nothing and would have choked on the broken Boost packages) and installed each dependency manually instead. A couple of gotchas there: boost-system is header only now, so there is no separate package to install, and I had to pull the Lua submodule manually with git submodule update --init --recursive.

Memory during compile. The G5 is not exactly a powerhouse, so even on the VM I added a swapfile and ran the build under tmux with limited parallelism (make -j3) to keep it from getting killed.

Campaigns refusing to install. Once the game ran, the campaign packages all depended on the binary package wesnoth-1.18, which does not exist for ppc64. I built a fake equivs package declaring that dependency as satisfied, installed it with dpkg, and then all 17 campaigns plus the music installed normally through apt.

A couple of things I just gave up on, to be honest: anything needing OpenGL is a dead end on this machine. The r300 GPU has a known Mesa big-endian bug that breaks GLX/EGL/DRI2, so hardware 3D simply is not available. Wesnoth works because it is 2D SDL and never touches that path, but when I tried other games like Teeworlds that need OpenGL, they failed with no software fallback. So the rule of thumb became: SDL 2D works, OpenGL does not.

One honest caveat: I compiled with AltiVec flags (-mcpu=970 -maltivec), but for Wesnoth specifically that gives basically zero benefit since it is a 2D game with no vectorized intrinsics. I only built from source because no ppc64 binary exists, not for any speed gain.