Is it possible to install Direct Access on a Server Core instance? I've tried looking through oclist and dism but to no avail.
Using a GUI instance just for DA would be a waste of resources IMHO...
If it's not specifically listed as a supported role or feature on the Server Core page, it's not supported. DA isn't listed so, no go. There are several rather slim roles that aren't supported that really bothers me but, such is life. go full version.
Needless to say, with some patience there are a great deal of things you could get working on a Server Core even if it's not supported, like all the parts of Exchange 2007... but I'd only do that in a lab for fun.
What do you need it for?
You could use SSH instead
The requirements for DirectAccess are here: http://technet.microsoft.com/en-us/library/dd637797%28WS.10%29.aspx