Multi-step PHP questionnaire in Drupal (for an expert system)

Without xstatic, is there any way of allowing POSTed data from a form to be passed through to a specialised script?

On our website, we need to build a small expert system. My colleague did this very effectively in PHP: it walks the user through a series of questions (difft paths depending on answer), before giving an analysis of the situation. After each step, the data is POSTed back to the script.

However, we couldn't see a way of doing this within Drupal, so the expert system runs outside Drupal (in a separate directory, with its own .htcaccess to prevent the request being thrown back into Drupal)

However, wanted it visually integrated with the rest of the site, so we eventually took the plunge and did a rather crude hack: used a few PHP include statements to load a header, masthead, footer etc to recreate the look-and feel of the main website.

Because we have only one theme, this works fine: we can't use multiple themes anyway, because our site is an informational one which doesn't allow users to login.

But it's a bit crude, and creates a few maintenance problems. Our custom theme is still under development, and when it is modified the included files have to be updated. This needs takes a little care because (for various reasons) we are not using base href in the expert system, so URLs have to start with root. Similarly, any changes in the main site's menu structure trigger a change in the include files.

So if it was possible without too much pain, I'd prefer to have the expert system properly integrated into Drupal.

It seems to me that the forthcoming xstatic module may provide a remedy, but even then I'm not sure -- I wonder if the POSTed data would be passed through to the expert system?

Is there any way of doing this without xstatic? Could it be done by writing a specialised module?