How to do URL Masking through web dispatcher,without changing output?

Hi,

I need to take an incoming Poral URL with embedded transaction code and convert it to an generic URL using Web Dispatcher, without changing the output.  example:

Incoming URL: http://<Hostname>/irj/portal/Ztcode/?ApplicationParameter=<invoice number>

Masked URL:   http://<hostname> or something else. The purpose to mask the URL is to avoid any user to change the invoice number in URL and look at invoice not intended for him.

The incoming SAP Portal link along with embedded transaction code with passed parameters would appear in external website and when the user clicks on that link, the invoice opens directly.
The application is WEb Dispatcher-->SAP Enterprise Portal-->SAP ECC server. No calls are made to SICF with our application, so we do not use sicf.

I tried using WEB dispatcher parameter icm/HTTP/mod_0 = PREFIX=/,FILE=\sapmnt\Filter_Rules.txt
RegIRewriteUrl (.*) /irj/portal [qsreplace]

But this switches the whole URl to http://<hostname>/irj/portal and in turn opens portal screen, instead of opening the transaction code with values. So i want to switch the URL but the output should remain the same.

Please help.

Thanks
Vikas