Rolling your own custom tasks provides you with the means to be as expressive in providing solutions to task automation needs as you wish. Grunt provides the scaffolding to get up and running quickly, allowing you to focus on building your plugin. It is highly likely that another developer has encountered the same type of problem and a solution already exists in the NPM registry. If not, then building your own is definitely within reach. It is a steep, but short, learning curve from entry to publishing your plugin. There is a great deal of information available as well as examples throughout the NPM registry and GitHub. Check out the gruntplugin tool at http://gruntjs.com/creating-plugins. Then, read through the generated concatenation plugin code and documentation to get started.
Have fun!