tl;dr: How do I do the following in a .conf or .htaccess file:
# Do A
# Do B
&
You can run different versions of Apache on the same host. Q: What's wrong with separate config files in separate directories? I honestly believe that's probably the cleanest approach...
Nevertheless, Apache .conf files allow an , which you can specify at runtime with "-D":
http://httpd.apache.org/docs/2.0/mod/core.html#ifdefine