You could use a language string to perform a substitution using the following example. You'll need to use the trigger onBeforeRegisterFormDisplay within your plugin. Then set the default field value for the email field to _DEFAULT_EMAIL.
Then write a plugin which executes the trigger on the following function (attached as a file due to forum security).
Attachment example_email_replacer.txt not found
Please note this is just an example and only 1 of the many ways to accomplish this.