Even if you could, I'd advise against it. WP relies on the slug being sanitized -- I suspect you'd break a lot of things if it was not.
The better solution would be to do one of these:
filter the_permalink:
Check for your page slug(s) and append the query vars to the end there, probably via add_query_var to be safe.
redirect:
Add some logic at the top of the page template (or filter template_redirect) to redirect to itself with query vars attached if the query vars are not set.
rewrite a custom url structure (may not work for your sitution, but I wanted to mention it):
e.g. /eventform/x/y/z .... => /event-name/?var1=x&var2=y&var3=z
WP Router makes this easier than dealing with the raw API.