Well, it might not really help you, but I encountered the same problem;
Here are 2 solutions that I used:
you own the website; if so, just create a test page that will simulate all the requests a normal page would do, and report a "TEST OK" or "TEST KO" as an output.
Then use the check_http plugin to poll directly this test page
That's a nice workaround the problem, which will help you greatly when trying to solve an issue on your website ... but you have to be the owner of the site!
ok; this solution is a bit of a brutal, but I like it:
just write a (big) piece of code in perl using a few plugins (such as a http or cgi plug in) to create a step by step test...
or, if you want, you can even use the telnet plugin and send yourself the GET and POST commands to your web server (that's the solution I prefer).
It would look like:
-create the telnet connection on port 80
-send the command "GET /index.php HTTP/1.1"
-read the result and check that a prerequisite sentence is in it
-send the command "POST /login.php?user=me&pass=my_pass"
...and so on
ps: if you don't know what answer to expect or what command to send to your web server, don't hesitate to install Ethereal : it's free and will help you greatly on the http client-server protocol!
I hope this will help