LoROM games such as Super Metroid previously had an issue reading any part of the ROM, other than the first 1%, when using RetroArch's READ_CORE_MEMORY API.
This issue is now fixed in RetroArch 1.10.1 and newer: libretro/RetroArch#13664
So QUsb2Snes should now be able to provide LoROM ROM data over the websocket when the game is running in RetroArch.