Set node version for non node web role

i have written an SPA which uses Grunt tasks to build itself from the source code into a dist directory.

I want to host this on azure to keep everything in one PaaS.

If i send the prebuilt dist folder to azure and add a .deploments task to move it into the DEPLOYMENT_TARGET folder everything works fine. However i want to just keep source code in GitHub and have the application built at deployment time using the existing Grunt tasks.

I have added a .deployment file with the following command

[config]
command = npm install

Which sucessfully triggers my npm install, and then the post install script

"scripts": {
    "postinstall": "node node_modules/grunt-cli/bin/grunt build"
},

which will use the local copy of grunt-cli (i don't like this but it works), however i get an error because the version of node being used is 0.6.20 and Grunt requires greater than 0.8

I have set my engine param in the package.json

"engines": {
    "node": ">= 0.8.4"
},

But no luck :(

I have a feeling that the engine param is ignored because a node application is not being found. Which is correct as i want azure to serve my files for me without using a node http server.

Is there a way to target another Node binary or specifically set the version of node for my website even if it is not "using" node as it were?

Any help greatly appreciated