Here's an idea that may work, depending on how the authentication is performed at the HTTP level of your web application. We support data driving load tests. The data source can be an external file (Excel spreadsheet, XML file, CSV file) or a SQL database. If you can come up with some mechanism whereby the required dynamically changing challenge data (e.g. string or number sent in text message to cell phone) is placed into the data source just prior to running the load test, then the load test can pull the data from your data source, use it in the authentication process and continue on its merry way.
If you use a SQL data base you could even go so far as to create a stored procedure that automatically runs on a regular basis to maintain the correct value needed by the load test.
Do you think this approach might work?
the Telerik team