Up till the latest version (v3-build-826-play), hi-res output sampling rate on my galaxy S8 relies on software resampler. All sources are resampled to 48khz (by default, or a specified rate up to 192khz) and passed onto the USB DAC. I've looked through the settings extensively and don't think native playback is supported. Selecting anything higher than 192khz only results in 192khz, while my DAC supports 384khz.
In contrast, Neutron player has long had the option called "direct USB access", and does output at the original sampling rate of the source without any resampling. The stream is sent directly to USB DAC for decoding. If I play a 44.1khz file, the DAC shows 44.1khz; if I then play a 192khz file, the DAC shows 192khz correctly.
It is clearly possible to implement this feature in android as Neutron has demonstrated. The OS does allow direct access to USB DAC despite fixing system sampling rate at 48khz. It would be a very useful addition to Poweramp.
An example of the desired behaviour is shown in the attached screenshot. With USB DAC plugged in, when "direct USB access" is enabled in the player, the OS prompts for permission. Once permission is granted, playback becomes native and volume buttons of the phone can no longer change the volume.