This question came up before somewhere and I answered with putting your Joomla root in a new directory and just having the splash page in the public_html directory.
Then everything else (including Joomla home item) would be:
thatsite.com/home/
Is there a disadvantage to doing it this way other than the extra directory name in the url?
I also mentioned this way as moving to any other server configurations would not cause any issues later on.
Probably best to use Directoryindex for this. Try adding the following line to your .htaccess file:
DirectoryIndex index.html index.php
This will basically tell apache to look for an index.html file first. If it doesn't find one, it will move on to the next file (index.php) and so on. You can add more options after that, so long as they're on the same line in the .htaccess file.