It's true - there are no limits in doing this but I don't think that's
what the question was necessarily getting at. If the question is
asking whether or not there is some sort of mechanism for adding links
to the 3D scene in some sort of HTML like manner, then the answer
AFAIK is no.
If you look at the "hud-2d-overlay.html" sample, that will point you
in the right direction. Essentially, you need to create a pseudo-2D
scene that is displayed on top of the 3D scene as many video games
would do - i.e. there is a separate scene graph of objects that is
rendered on top of the 3D scene using an orthographic camera to make
it look 2D. You can then set the event callback for the mousedown
event - which is set for the main client object. Inside this callback,
you then have to go through the relevant clickable objects in your
scene, doing hit tests against them and responding as required.
Basically, you have to take a fairly brute force approach to doing
this.
I had to do this recently - I wanted to add 2 or 3 buttons on top of
the 3D window and was hoping that it could be done entirely in HTML
with some simple javascript calls in the onclick handlers - but, alas,
no. Even though taking the approach I mentioned above is fairly
painless, it does involve a lot more code than
<img onclick="viewer.DoThis()"/>
which is a real shame.