Hello Matthew,
GetCustomActionURL returns an URL, and GetHyperlink does more than just creating the HTML syntax, so I suggest you create the link in HTML directly.
So, for example, if you want to define a link in HTML format, you can use something like:
<a href="{GetCustomActionUrl('Admin Project Closure',GetRuntimeParameter(CurrentObject()))}">Click here to run Admin Project Closure</a>
As you already use HTML tags, I suppose you are writing an HTML page (or email) from a Workflow rule, so I'll suppose you're familiar with HTML syntax.
I hope this helps