I found a MPEG 1 Audio Layer 1/2 encoder, still in the Debian repositories: mp2enc included in the package/compilation mjpegtools. Mostly sample code, but could be a fast solution for the upcoming release.
mp2enc -l 1 -b 384 < /mnt/c/Users/Max/in.wav -o "out.mp1"
INFO: [mp2enc] Opened WAV file, freq = 44100 Hz, channels = 2, bits = 16
INFO: [mp2enc] format = 0x1, audio length = 39321488 bytes
INFO: [mp2enc] SpF=96, frac SpF=0.000, bitrate=384 kbps, sfreq=48.0 kHz
INFO: [mp2enc] System is little endian
INFO: [mp2enc] Num frames 27863 Avg slots/frame = 96.000; b/smp = 8.00; br = 384.000 kbps
INFO: [mp2enc] Encoding to layer 1 with psychoacoustic model 2 is finished
INFO: [mp2enc] The MPEG encoded output file name is "out.mp1"
It does only support 8 or 16 bit depth, 18 or 20 bit as discussed will require massive extra work, as it is not even common for PCM: 18 bits recording. How to put in practice?
They are quite honest in the manual:
It is actually a very mildly warmed over version of the MPEG Software Simulation Group’s reference encoder.