-The site with the issue is a Wordpress installation.
-I have included the following (see code below) within the root directory's .htaccess file (both before and after WP's mod-rewrite lines).
-I have also tried including it within the /fonts directory's .htaccess file.

I am experiencing the following behavior in IE9 ONLY (all other browsers get the green light): On first page load for any non-primary domain, the fonts do NOT render, and developer tools report "@font-face failed cross-origin request. Resource access is restricted" and similar. Upon refresh, all fonts load with no issues.

I understand the issue; what I do not understand is why the following code would fail to modify the headers to allow the font to load the first time in, and succeed thereafter:

 <FilesMatch "\.(eot|otf|woff|ttf)$"> 
 SetEnvIf Origin "^http(s)?://(.+\.)?(domain1\.com|domain2\.net|domain3\.com)$" origin_is=$0 
 Header always set Access-Control-Allow-Origin %{origin_is}e env=origin_is 
 </FilesMatch>

*Please note that between each change to the .htaccess file, temporary internet files were cleared.