Should be possible to store digital music on cassettes as well just like you would with a tape backup. Would probably increase both the quality and storage capacity of cassettes.
According to that article, it looks like the best cassettes (if everything goes right) could have a bandwidth of 20kHz. So a quick calculation of 8bit samples: (20000 * 8) / 1024 == about 156kB/s bitrate. If i did my math correctly, a 90 minute cassette could keep 824 MB (raw)
The article suggests that they struggle to sustain 20kHz, and I assumed you could keep 8 bits per sample cleanly, but the actual data size would probably be much smaller than that. Keep in mind, besides the mechanical wear and tear, seeking in the digital data would be tricky, the data stream would have to lose some of that raw data size for framing + synchronization marks.
They did have to work with any crap tape recorder and the interface circuitry wasn't much.