I am trying to assign a variable either from my template file or otherwise which I can then access within the post editor.
There is no built-in mechanism to access php in the post editor. The best solution would be to create a custom shortcode which you can use to access the variable. This is probably the safest and most secure method available.
You haven't been too descriptive but if you mean that you would be able to edit a variable in the Post > Edit screen then you should really be using Custom Fields. These variables would be stored / retrieved as post meta-data.