trouble with byobu custom status

I'm trying to add nVidia GPU utilization to my byobu-tmux status line. I've installed gpustat which returns

~$ gpustat
myhost                 Mon Aug 19 17:26:51 2019  410.48
 GeForce GTX 1080 Ti | 22'C,   0 % |     0 / 11178 MB |
[1]  GeForce GTX 1080 Ti | 41'C,   0 % |     0 / 11175 MB |

then I wrote a text processor saved to ~/.byobu/bin/2_gpu as suggested in the byobu web page for custom status.

~$ cat .byobu/bin/2_gpu
#!/bin/sh
echo "$(gpustat | tail -n+1 | sed -nr "s/\[[0-9]\].+([0-9]+)\s\%.*$/\1%/p" | tr [:space:] ',' | head -c-1)"

from the bash shell that seems to work; whether the shebang is sh or bash doesn't seem to matter, it works fine:

~$ .byobu/bin/2_gpu
0%,0%

however I can't get that text onto the byobu-tmux status line. A simpler custom status works fine, but there's some problem escaping the quotes and/or '$' in the regex argument to the sed command. I'm not sure why echo is even needed, but removing the echo "$(...)" doesn't help, nor does adding backslashes on the inner double-quote and '$' characters.