Binding to JNDI datasource - please help

Hi all, I'm relatively new to JNDI concepts and need some help.  I am
writing a web app that accepts a variable specifying a jndi pooled
connection datasource as a url variable.  This is a requirement and
cannot be changed.  For example index.jsp?db=java:defaultdb

How do I use this JNDI name to create a connection?  All the
literature I have read says to bind the resource manager connection
factory reference (res-ref-name) to the (jndi-name) in the server
specific xml (assumes knowledge of jndi name beforehand).  Then do a
lookup on res-ref-name.  In this case I do not know on what server
this war will be deployed and want to bind directly to the datasource
defined by my url variable.  Can I do this without using a
res-ref-name and doing a lookup?

Your help is apprecitated,

John