Changing the URL wouldn't be that simple, you would have to rename the file(s); but this would not gain you an advantage as the user can still manipulate the "id=x" portion of the address
Machform loads the *form* itself from a database (not just the data submitted)therefore it requires the id=x appended because it's SQL query works of that, to load the correct form.
I think in the next version it will be coming out with some more advanced password protection, multiple user type features, so this might be the way around your issue.
What is the cause of your concern regarding the user possibly changing the id to something else?
I just tried the service http://tinyurl.com/ in the offhand that it might be what you need but it merely redirects to the actual URL and doesn't mask it from the user (therefore they can still change the "Id" value).
For example, take a look here: http://tinyurl.com/6wv9hh (click this link) and see it redirects to my Machform form with the actual url of http://www.murraywebs.com/machform/view.php?id=18 - without actually displaying the long URL.
Edit:
On the other hand, I just tried it and it doesn't do what I thought; it just redirects literally to the long URL. "Tiny URL" is just so you don't have to continually enter the long URLS with all those sort of codes you typically see on some database driven sites (like Microsoft).
One way to mask the URL is to use a frameset, but frames are deprecated technology and not really current as far as modern DocTypes and Browsers are concerned (an also cause problems with search engines).
So my answer probably hasn't helped, sorry.....