Yah there are some limitation on the browser forms but I may have a solution
for you.

Check out the Database Accelerator download at http://www.qdabra.com.

It has web services that allow you to perform multiple queries and submit
data to a SQL database, even with a browser form.

So you can have the first dropdown trigger a query of the second table for
the selected value and return the data for the drop down, which will give you
the cascading you want. And then you can send all of the data in the form to
as many table as you want.

Contact me if you need assistance or a demo.