The miniupnpd package has been constantly updated, but new functionality has not been coded to be exposed to uci and luci-app-upnp.
STUN functionality can work, but you'd basically have to allow for all UDP packets to go through your firewall (which is bad).
Currently, there is no maintainer for the miniupnpd package - only volunteers who have been updating the package. Besides, I don't think anyone would really want to look at changing the config files until a version 2.2 comes out and everything gets more stabilized on miniupnpd's side.
You could try and just use an older version of miniupnpd (e.g. 2.1), before the introduction of the STUN mechanism. But then, you'd have to build an OpenWrt image yourself.
Looking at Gargoyle, their current master branch/v1.11.0 uses miniupnpd 2.1. Thus, you will be able to then run your DMZ configuration without miniupnpd acting "smart".