Try to just use a index.html file and use
<META HTTP-EQUIV="Refresh" CONTENT="5; URL=http://example.com/wp-content/themes/your_theme/index.php">
in your <head> section.
Let’s see if the easy stuff works.
edit: to be on the save side, always display a link to the index.php file with some descriptive text like “if your browser doesn’t redirect you automatically, please click here“