Hi Hutch,
How about you do this:
1 - On 3E you could setup a temporary domain like myclient.yourdomain.com and develop the new DNN site there;
2 - Once the new site is finished, just add your client's domain as a site alias ion the new site;
3 - Then you could copy the site files and database and ask PowerDNN to move the new site files over to replace the old site;
I hope this makes sense to you.
Cheers,
Aderson