I thought that it wouldn't be a problem, because of the routers wiki stating this about partitions:
Additionally, the stock partitioning assumes a ~35M size for each of the `rootfs` MTD partitions and ~30M for the `overlay` MTD partition. Even with that, the mounted `/overlay` partition only has a size of 15M.
OpenWrt partitioning modifies this, allowing the use of all available space, so it is no longer a problem. This partitioning will be reverted when recovering the router to the OEM firmware.
But I guess I won't flash until someone can confirm it's safe.
Get it back into factory state, with the original partitioning, go on from there. OpenWrt hardcodes the mtd partitioning, and if that doesn't match expectations, it'll be bricked for good.