Very interesting question Troy. Kind of like a chicken and egg situation. You build a mashup from Qlik Sense applications that you want surfaced by Qlik Sense.
As far as I know you can't have a mashup shown in the "hub" itself. HOWEVER, there is no reason at all that you can't create a Qlik Sense Application called MASHUPS that would be visible on the HUB that everyone could access.
There is a wonderful little extension called 'sense-media-box' available on Qlik Branch (Branch.Qlik.Com) link follows: Click here to jump to the Sense-Media-Box project so you can download it.
It allows you to do several cool things like embedding HTML content and Web Pages. Let's say for instance you just can't live without having access to my Qlik Dork blog inside every single one of your applications. You would simply add the Media Box extension to a page, then choose "Web site" as the MediaBox type, slap "http://QlikDork.com" into the Web site (Url) and be off to the races. Then on another page you could add another media box and use it for your mashups URL location. (Hope that comes across more as humerus than totally self serving)
But wait ... there's more. If you act in the next 30 minutes and are one of the first 8,309,301 callers you also have other options for embedding "Mashups" inside of the Mashup application. Because the Media Box also enables pure HTML code. Seriously! You know that stuff that you build mashups with ... No reason you can't use the Dev Hub to pull the IFrames for objects from other applications and then bam slap them inside your Qlik Sense "Mashup" QVF.
For only the price of shipping and handling ... let me throw in yet another use for the Media Box control ... a web service that enables you to post comments/annotations within the application. Ok. I realize I took a left turn on you with that, but "Hey you got me talking about the Media Box and it just jumped off my fingers."
In the end Troy this may be a long winded answer to your question as you really just wanted the mashup shown right in the Hub, not another application. But hopefully if it doesn't resolve your question, it will spark the dreams and imaginations of others.