What about on an Android phone or an iPad? Our application runs in the mobile browser on these devices. I'm not aware that there is a Unity3d plugin available for mobile browsers. I'm also not aware of any way to start an app from a web page. Is there any way to start a Unity3d app from a mobile browser?
Any suggestions welcome.
You're right in assuming that Unity3d doesn't have players for mobile.
I don't know how to do this for Android but for iOS you can build the Unity project then open up the xCode project that it created and add a custom URL Scheme. This tutorial explains how to do this.
After all this you can create a link in your browser page that uses the URL Schema you created.
<a href="myunityapp://something">Go to Unity</a>
Note that you need to have a URL Schema that is unique otherwise Safari can open other apps that have the same Schema registered.