Provide option to specify compression options

It is possible to select one of four pre-configured compression options via -z. I am wondering, whether it would be a useful thing, to extend this option. In particular, I'm interested in allowing to pass options in a profile. In my case, I want the fallback image to get compressed with xz -9 and the default image to be compressed with zstd. (Reason: My /boot partition is too small, because I have a Dual-Boot with Windows 10.) I can pass -z xz in fallback_options, but I cannot do something like -z 'xz -9'.

I could imagine to

1. pass a separate option for compression options (seems to be a better option)
2. allow to pass an option like -z !'xz -9 -v', which would get interpreted as "parse the part after the ! as string, and use that as compress command -- and use at your own risk.