I think in site.ini there is a setting for this. Under
[SiteSettings] you have
# Which page to show when the root index (/) is accessed
IndexPage=/content/view/sitemap/2/
# Displayed if an error occurs and ErrorHandler is set to defaultpage DefaultPage=/content/view/sitemap/2/
This is set per logical site so changes should go to /settings/siteaccess/(yoursite)/site.ini.append(.php)
Shouldn't change too much in the PHP files. This'll be a mess for upgrading. :-)