I'm sure I could add the code (and support it for a short time) fairly cheap (<$50) As mentioned previously, however, when updates to DLGuard are released the changes could be overwritten and need to be re-inserted.
In designing and developing ListMail I try not to modify the structure of the "users" table so-as not to break 3rd party plugins. Still, I can't guarantee that the table will not change in the future, as I remove limits and improve efficiency, but there is a 99% chance it will remain the same.
Both programs can change and therefore, unless DLGuard was to create and maintain a plugin, if either one changes, the process would need to be updated. To avoid this it makes sense that a plugin be created for the membership software, because it handles the insertion. They update their program, so when making updates they can make sure the ListMail inclusion/plugin routine is called and passed the correct data.
Apart from the addition to their interface it's a simple matter really. To add a user to a list/followup sequence (no welcome email - that's a bit more dfificult) all it takes is a single MySQL insert statement.
Regards