Thanks for posting!
The iwp_auth.html and iwp_home.html pages can be found in the following directory:
C:\Program Files\FileMaker\FileMaker Server\Web Publishing\publishing-engine\wpc\Resources\iwpres\
/Library/FileMaker Server/Web Publishing/publishing-engine/wpc/
Control + Click on FM Web Publishing -> Select Show Package Contents
You can edit these files to change what's presented to the user.
In addition to modifying the iwp_auth.html and iwp_home.html files, you can also edit the label, to change the header that reads "Instant Web Publishing" as well as the web page title, i edited the file stings.js using a text editor, the path to the file is here:
c:\Program Files\FileMaker\FileMaker Server\Web Publishing\publishing-engine\wpc\Resources\iwpres\eng\strings.js
the file list the strings to edit (among others):
home_ttl_banner: "Instant Web Publishing",
home_ttl_iwp: "Filemaker Instant Web Publishing",
Once edited it displayed as desired.