I don't use jekyll and have a fairly limited knowledge of pages but doesn't jekyll output static html? Even if you used a dynamic language to create the project, wouldn't it not matter because the end result will be static?
You'd be able to run arbitrary Ruby code during the compilation of your site. GitHub uses Liquid templates as a pseudo-sandbox: it's deliberately a weak system to prevent abuse. But it's lazy, because there are many other solutions that don't take away from user experience.