What do you mean by “OSMC filesystem is created”?
Is this only for a “new” or “clean” install from SD card? If it doesn’t apply to upgrades via “apt-get dist-upgrade”, then it’s no big deal once we have the man pages. A separate “man-omsc” package would make getting them in the first place faster, though.
On the other hand, any update system that nukes existing data files (which is exactly what man pages are) on the disk is broken, and needs to be fixed. A way for a user to circumvent such a delete would be to “cp -a /usr/share/man/*” to a directory on the removable micro SD card, then bind mount /usr/share/man to that new directory, but mount it read only by default. Then, an upgrade can’t nuke it.