So Peter helped me write this little Perl script, which does that. It's idempotent, meaning you can run it multiple times, and it won't mess up.
perl -pi -e 's#([^=])(workflow::[a-z_:]+)(?=[^a-z_:])(?!</a>)#\1<a href="/api-doc/proc-view?proc=\2">\2</a>\3#g' developer-guide.html
You'll need to modify it for your own needs. I'm searching for 'workflow::' here, only care about lowercase, and I deliberately only process the developer-guide file, which is in HTML.
It would be good if our docbook XSL contained a tag for API procs, which would do this.