What is your naming convention for your submitted XML?
I ask because you may be able to use a formula to create the URL you need - you could set the default value for a field to the formula for the URL and then have a hyperlink control on your form with that field as the data source....
The only real hiccup would be if your file names contain characters that SharePoint changes because they are illegal. If so, you may have to get a bit fancy with the "translate" function.
Are you using IP 2010 / ShP 2010? If so, you can create a data connection to the library you are submitting to and indicate that you want data only for "this" item - just a guess here, but I'm thinking if you submit via rules, you could submit to SharePoint, then query with the data connection to the library to get the actual XML file name, use the result in the formula for your hyperlink, and finally submit via email.... Mind you - I've not tried that, I'm just thinking outloud.