Well, the underlying filesystems are now full enough here that I can see that mhddfs actually is working to distribute files across both arrays. So perhaps there isn't "a bug in there" after all!

Here's how both of the filesystems get mounted here.
First, the main Mythtv storage heirarchy. For this, filesystem, capacity balancing isn't forced on until a drive dips below 400GB free.

/usr/bin/mhddfs /drive[123] /drives -o allow_other,auto_cache,max_write=4194304,uid=1000,gid=1000,mlimit=400G

And here is the backup array, mounted only when needed. Capacity balancing is set to kick in even later here, at the 200GB per-member threshold:

mhddfs /.bkp/drive[1234] /.bkp/drives -o rw,allow_other,auto_cache,max_write=4194304,uid=1000,gid=1000,mlimit=200G