How to pass a parameter/argument to webform using clean urls?
You could solve this using mod_rewrite in your .htaccess file. Try something like 

RewriteCond %{REQUEST_URI} ^/path/to/your/webform/(value)$
RewriteRule ^.*$ /path/to/your/webform?param=%1 [L]


This way you can do many things but you can't configure that with the drupal UI. Here's an example with more paramters:

RewriteCond %{REQUEST_URI} ^/path/to/your/webform/(value1)/(value2)/(.*)$
RewriteRule ^.*$ /path/to/your/webform?param1=%1&amp;param2=%2&amp;param3=%3 [L]


Check out the manual.

EDIT: your comment example should look something like that:

RewriteCond %{REQUEST_URI} ^/myform/(\d+)$ 
RewriteRule ^.*$ /myform?i=%1 [L] 


The exact regex at the end depends on your parameters. If you expect only numbers like 1 or 0125 \d+ should be fine. Check out this cheatsheet for more/other char-classes and regex stuff.