Got a specific patch in mind, or just a general "this would be good" feeling? ;)

It was definitely more than a feeling :)

I'll attach a patch showing my modifications.

Created attachment 19340 [details]
The patch introduces an invoke operation to the proxy

The operation is specified by the "op" parameter and the parameters of the JMX
operation are specified (as a comma seperated list) by the "ps" parameter.

This Tomcat 5 enhancement request has been moved to Tomcat 7 (the latest version) since Tomcat 5 development is limited and focussed on bugs and security issues whereas Tomcat 7 is still seeing new feature development.

After rather too long a variation of this patch has been applied to trunk and 7.0.x and will be included in 7.0.24 onwards.

The main changes I made to the proposed patch were:
- fixing a potential NPE when a method required zero parameters
- improving the output when an array was returned (as is often the case with Tomcat MBeans)

Very useful feature and i'm just putting it onto our tomcat 6 boxes. However it doesn't account for the invocation of the mbean returning null when outputting the result. I propose that the invokeOperation becomes

    public void invokeOperation(PrintWriter writer, String onameStr, String op, String[] valuesStr){
        try {
            ObjectName oname=new ObjectName( onameStr );
            MBeanOperationInfo methodInfo = registry.getMethodInfo(oname,op);
            MBeanParameterInfo[] signature = methodInfo.getSignature();
            String[] signatureTypes = new String[signature.length];
            Object[] values = new Object[signature.length];
            for (int i = 0; i < signature.length; i++) {
               MBeanParameterInfo pi = signature[i];
               signatureTypes[i] = pi.getType();
               values[i] = registry.convertValue(pi.getType(), valuesStr[i] );
           }

            Object retVal = mBeanServer.invoke(oname,op,values,signatureTypes);
            if(retVal != null){
                writer.println("OK - Operation " + op + " returned:");
                output("", writer, retVal);
            }else{
                writer.println("OK - Operation " + op + " without return value");
            }
        } catch( Exception ex ) {
            writer.println("Error - " + ex.toString());
            ex.printStackTrace(writer);
        }
    }

Created attachment 28173 [details]
2012-01-19_tc8_JMXProxyServlet.patch

Yours covers most common use case of method returning void/null.
I think null values inside arrays should be handled as well.
I am attaching patch that I am thinking about that covers null values in getAttribute() operation as well.

Patch looks good to me. Applied to trunk and 7.0.x and will be included in 7.0.26 onwards.