Should my mythtv users has the same UID on core and MD's? they don't currently. also, my MD doesn't have mythtv user as part of the mythtv group, or as part of the public group. this seems like it might be part of my problem as linuxmce overrides the usual mythtv permissions (mythtv:mythtv) with root:public. myth-backend is running at the mythtv user rather than root, so it seems important that mythtv user be part of the public group in order to inherit write permissions!!
now, it seems odd that things are working at all.