What I'm looking at is my client has their main website. When a new user enrolls on their website, and pays the required subscription fee, they want the client to receive their own personalized website.

I know JMS could create the website, subdomain, database, etc. I also know that Joomla! can create a new contact based on the registration.

On the main menu if I have a menu item for contact me, could JMS dynamically change that contact from the default to the contact for the client who's site is being automatically created?

Basically, the default contact on the menu item is Master User. The client signs up, with the name being New User. Upon creation of the site, the contact menu item would change from Master User to New User, so the client has to do absolutely nothing on their site.

Yes it is possible to perform customization of a website when it is created.
This is possible when you enable the billable website feature that allows executing additional plugins. This billable website functionality require website quota. Go in your JMS setting to see the quota management.

When you create a JMS layout for the website creation form the front-end, you can add custom fields that you can use later by a plugin.
For some customer, we already created such kind of plugin.
The solution that we created is generally based on the Search/Replace plugin that we automatically configure based on fields present in the layout.
So that, you can use keywords anywhere in your website to be replaced by the real value.

If you want that we create such specific plugin for you, you can order that at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1