There's already a textmate bundle: https://github.com/Anomareh/PHP-Twig.tmbundle
It's possible to convert textmate bundles: http://ace.ajax.org/#nav=higlighter (look for "Using the TMLanguage Tool")
I have not done this yet but it should get you 80% of the way. It would be awesome if Ace added it but Twig is likely too obscure for them to offer oficial support.
+1 would love to see ace support twig templates!!
This pull request by @nightwing includes an HTML Twig mode. Apparently he converted a large handful of modes all at once. You could merge this pull request into your own branch or you could wait them to merge it here.
Also, I created one for my own purposes but you have to build Ace yourself to use it. It's been working well enough for me but it's probably not perfect.
@heygrady i tried to convert Anomareh/PHP-Twig.tmbundle but it required too many manual tweaks, so i had to remove it ab1ed57.
Maybe you could make a pr from your ace-mode-twig?