I'm very reluctant to go drilling 25mm holes through joists to fit tubing just to thread a 4mm fibre. The conduit does nothing to add mechanical protection.You can try it but they may not want to run the fibre in with a bunch of cables in case of mechanical damage. Is there room for the green tube that they use to protect the fibres outside?
Bottom line is this thread has given me areas to research and it'll be down to a conversation with the installers whether I go ahead or not.