I don't think that will work.
Given there aren't any user space utilities I'm guessing the kernel is expected to do this but it looks like the kernel code won't touch anything that looks corrupt so I think it has to be corrected manually.
However, it also looks like (although I'm not sure) the kernel will be happy to do it if it looks like it's been erased, whether that's all F's or all 0's I'm not sure, and I don't know if the mtd-erase2 leaves block headers in place and erases the block contents, again a user space manual file system create might help.
Or maybe a dd of a copy of an empty jffs2 partition,
It's not done by any userspace tool, it's done by the rc daemon using lowlevel system calls. Same way it has always done, regardless of the state of the JFFS partition.