Embeddable Modules header access

How can one add code into the header for Embeddable Modules?

That's something unusual, considering that embeddable modules should be... embedded.
But I guess all depends on what the final goal is. So let me ask some questions.

I suppose this is about invoking the RTD module standalone, is this right?
And when you ask about "code into the header", what do you mean? HTML, Javascript, or something like Meta tags?

To use google web fonts, code needs to be in the header.

Not 100% true.
The link tag can be used also inline. For instance, you include google fonts with something like this:

<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,800,300,700' rel='stylesheet' type='text/css'>

In general, it's recommended into the head section (not the header), but simply because allcontents before it would start to be rendered otherwise.
However, all you need to do is to add that LINK tag on the very top of the 1st variable you're using.
In short, placing it inline in the RTD, but before other contents