It's possible to do some supported customization of the logon page, but nothing else, I'm afraid.
If you want to do other customization, there are ways to do that too (you'll find everything on your server here: C:\Program Files\Windows Server\Bin\WebApps\RemoteAccess\Customization) but you should be careful what you change and how, lest you render your
remote access site non-functional.
I'm not on the WHS team, I just post a lot. :)