Placing the module in the Documents folder in my home folder for signing has solved this problem! And apachectl is happy if I run the config test (after placing the signed module back in its place). Apache still doesn't start, but that's another problem, this one has been solved. Thank you so much for your help

I have since installed Apache successfully, so for anybody else who's having problems with Apache: although this post shows how to properly sign a file with a developer certificate, in actual fact it's pointless for Apache. The secret of installing Apache on M1 is to start from scratch with a new installation using either MacPorts or Brew. Because installations that have been done on your previous macs and copied onto your new M1 will only give you big problems, because the installation will be on a part of your file system that has been protected by SIP, which is impossible to work with. Whereas Apache installs from MacPorts or Brew are installed on read/write parts of your file system and are freely configurable. You will still find a few problems on the way but they are definitely surmountable, and signing mod_jk.so will not be necessary

If anyone is interested I have detailed everything I did to get Apache installed in a stackoverflow post: https://stackoverflow.com/questions/70283661/how-do-i-compile-tomcat-mod-jk-on-a-macbook-pro-m1-chip/70852729#70852729