I've at last got found a solution to this problem, using a GigaBlue UHD Quad 4K. In its Video & Audio Settings menu, AAC Downmix must be set to Passthrough -- but in addition, PCM Multichannel must be set to Yes. If you then tune to BR-Klassik (12265 MHz H, 19.2°E) and use the Audio button on the remote control to select AAC-HE Surround, you should hear the test stream, a recording of Beethoven's Leonora No 3 overture in 5.0 sound on a loop -- either that or an actual broadcast.
The two other receivers I've been able to try don't do this. My Vu+ Solo 4K has a shorter Video & Audio Settings menu which does not in include the PCM Multichannel setting. Its Audio button brings up the AAC-HE Surround stream option, but selecting it produces only silence. On the Octagon SF4008, the Video & Audio Settings menu is even shorter -- it does not have an AAC setting, although the receiver happily plays the AAC stereo sound on French TV channels and Hungarian radio. If I select the AAC-HE Surround stream on BR-Klassik, it plays, but only as a stereo mixdown.
All these receivers are running OpenVIX v5.4.016.
Presumably there are other receivers which can play AAC surround correctly, but I can see no obvious way of knowing which.