In order to have a single-instruction PR-relative load the offset needs to be small (though it uses a different immediate scheme for the offset). So in practice you usually tuck small constant islands between blocks of executable code when you adopt this approach.
That's what I was thinking. I also started wondering how a large structure might be packed, when you can relatively address every byte in the nearest 256 bytes but only every 4 bytes in the next KB and so on. It could sort of upend the way you might normally think about packing a struct.