How can I implement a global RewriteCond / RewriteRule in Apache that applies to all virtual hosts?

Specify RewriteOptions InheritDown in the parent scope (such as httpd.conf) to get your rules applied in child Virtual Hosts without modifing them.

This will only work on Virtual Hosts where the RewriteEngine directive is set to on:

Note that rewrite configurations are not inherited by virtual hosts. This means that you need to have a RewriteEngine on directive for each virtual host in which you wish to use rewrite rules.

(source)

Apache supports this since 2.4.8 (not available at the time of the original question).

From documentation for RewriteOptions:

InheritDown

If this option is enabled, all child configurations will inherit the configuration of the current configuration. It is equivalent to specifying RewriteOptions Inherit in all child configurations. See the Inherit option for more details on how the parent-child relationships are handled. Available in Apache HTTP Server 2.4.8 and later.

InheritDownBefore

Like InheritDown above, but the rules from the current scope are applied before rules specified in any child's scope. Available in Apache HTTP Server 2.4.8 and later.

IgnoreInherit

This option forces the current and child configurations to ignore all rules that would be inherited from a parent specifying InheritDown or InheritDownBefore. Available in Apache HTTP Server 2.4.8 and later.

(http://httpd.apache.org/docs/current/mod/mod_rewrite.html#rewriteoptions)

By default, mod_rewrite configuration settings from the main server context are not inherited by virtual hosts. To make the main server settings apply to virtual hosts, you must place the following directives in each <VirtualHost> section:

RewriteEngine On
RewriteOptions Inherit 

click http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html to find more information

Looks like the simplest possible solution is to add

RewriteOptions inherit

to each VirtualHost directive. This is at least a lot simpler than messing with .htaccess files. Apache is pretty clear on the fact that

by default, rewrite configurations are not inherited. This means that you need to have a RewriteEngine on directive for each virtual host in which you wish to use it. (http://httpd.apache.org/docs/1.3/mod/mod_rewrite.html)

and apparently the way to change the default is via RewriteOptions in the child (vhost or director), so you have to do something in each child.