I'm have a custom plugin that I have developed for my solution and I'm in the process of extending it and doing a few more things inside of the plugin as opposed to inside of FMP as I had been doing before. However, I need to be able to access several variables which my function generates and I'd like to know if there's essentially a way to bind the SMGetVariable function into my custom plugin as opposed to also including the ScriptMaster plugin with my solution?

If I understand your question, then no. You cannot embed a plugin's functionality into a custom function in order to get rid of the plugin.  Plugins are written in C or some other language and their functionality lies outside of the realm of what is available from within FileMaker. They extend FileMakers capabilities.  Custom Functions simply make use of what FileMaker already has available to it.

 

What is the purpose of using a plugin to store an external variable in your situation? The only real reason to use an application/plugin level variable is so that the result can be shared across multiple open files.  This can be overcome if you are able to fire off scripts in the other files in order to populate file level $$ global variables.  There are custom functions that can fire off scripts when referenced from within a calculation which can make your life easier if you are trying to have everything happen in a single call.