Wallpapering - filling space underneath paper?

In the past I've managed to remove painted anaglypta from the boxing around old pipes, remove the boxing and pipes and re-fix the paper. A wipe of filler over the joins and cracks, a lick of paint and it was invisible unless you were really looking for it. You might be able to do something similar - slit down one side of the hollow bit with a sharp knife and peel the paper back a bit, squidge some Polyfilla in the gap with your thumb then gently press the paper back down so it meets up again. Wipe down with a damp cloth once the filler has firmed up a bit, then paint over when it's dry. You'll only have to do this where damage is likely to occur.