I raised an issued on Twinaphex's Github a few weeks ago regarding loading PWADs, DEH etc.
I think they are putting all of their effort into getting the Android version working.
The port of Star Wars Doom is able to read PWADs and DEHs (at least the ones that come with it). Looking at how they did it might shed some insight. But it is based on PrBoom Plus, and like you said the RetroArch team seems to be focusing on other things right now.
Apparently, the standalone version also reads PWADs, but I have an older version which doesn't, and I can't find the newer version anywhere (it's been removed from Mediafire). Oh, well.