I'm not sure why I didn't do this before -- it's clearly much simpler to add dumping of the extra thing than to have it as another implicit source.
Even without any clever optimization on the unpack operations, this gives us a useful value for the channels read field, which we can use to avoid ldtmu instructions to the no-op register. instructions in affected programs: 890712 -> 881974 (-0.98%)
V3D 4.x texturing changes enough that #ifdefs would just make a mess of it.