You need to take a step back and think about what you're asking here.
First, the 'codebase' line you're referring to is an attribute of an 'object' tag. That tag, in turn, is an ActiveX command that will ONLY be understood by Internet Explorer and ONLY on Win32. So, you've no need to worry about Mac/Linux users
(as an aside, the original ActiveX downloading spec allowed you to specify different ActiveX binaries for different platforms, but seeing as how Microsoft have discontinued IE for Unix...http://www.microsoft.com/unix/ie/default.asp
...I don't think this will be a problem going forward!)
The 'embed' tag (which only Netscape 4.7x understands) DOES have a 'pluginspage' attribute which will automatically redirect the user to 'java.sun.com/getjava' if you want (not in the example given by ap_kelly, but I'm sure you can tweak that).
Outside of the 'automatic downloading' world, you can simply offer a link to 'Get Java' (Sun has a Java Banner Program you can sign up for), much as you would Shockwave.
Personally, I'm not a big fan of the converter either. A much better way is to use the standard JSP taglibs (they have an 'embed applet' tag). However, neither of those produce as robust a solution as is discussed on...http://java.sun.com/j2se/1.4.1/docs/guide/plugin/developer_guide/using_tags.html
...(which features my favourite quote 'Although this tag seems complicated compared to the old APPLET tag, it is not.'), so you can always implement your own JSP taglib to do that.
In fact, if JSP is the way to go for you, I can post such a taglib here.
As proof all this really does work, please feel free to check out my site at...http://www.juliannegiffin.com