But XenForo isn't a whole separate application? I understand what you want, but take a deep breath and think if you really need it. IMHO you're complicating thing for yourself. What if XenForo use some magic URLs that points to non-existent files?!
I'm not able to point all possible complications, but surely there will be some of them, and you can avoid this by simple put XenForo installation anywhere than the folder that Phalcon uses for their own assets. Each project has their conventions, I'm just have a bad felling about this :)
Here's my suggestion: You can remain with XenForo inside of 'phalcon' folder but move it outside the 'public' folder and add some rewrite for this folder in the main .htaccess
:
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^$ public/ [L]
RewriteCond %{REQUEST_URI} !^/forums(\/.*$|$)
RewriteRule (.*) public/$1 [L]
</IfModule>
Basicaly the same thing you're doing before, but in a more "safe" place.