Not sure if you played around with this, but I would restrict the ARC cache to a certain amount of memory (like, 100 or 200MB). It's not a real hardlimit, si it can go higher, but it would level of around this point. You get kernel panics when you are memory constraint and you do not limit the ARC.Further, I have discovered that while I can access this external drive, some copy operations cause my Pi to completely hard crash. For example, I can copy any one file I like, but copying a whole directory (ie cp * /to/destination/) brings down the operating system.
Second update: I have found I am able to create/move/delete many files at once on either my ZFS volume OR the UFS partition. The crash only happens, it seems, when I try to bulk copy files from one file system to the other, in either direction. I can, for example, unpack the ports tree or copy the kernel source code around to various locations, so long as I stay within the boundaries of the respective file systems.
It may occur more often when you cross-filesystem copy your data, I imagine reading in UFS and writing to ZFS may wreak a little havoc in cache use.