Can you really result in the DirectoryIndex value inside a .htaccess file conditional according to IP, to ensure that - for instance - my IP see's DirectoryIndex as index.html and everybody else sees DirectoryIndex as index.php?

It is possible to solution apart from mod_rewrite?

So far as I understand, there's no conditional for DirectoryIndex. You can simulate by using a mod_rewrite directive like that one:

RewriteCond %{REMOTE_ADDR} your_ip
RewriteCond -d
RewriteRule (.*)/$ $1/index.html

If you wish to exclude other site visitors from the site from viewing index.html then also employ

RewriteCond %{REMOTE_ADDR} !your_ip
RewriteRule (.*)/index.html$ $1/index.php

While using provided information I beleive this is the thing you need:

RewriteCond %{REMOTE_ADDR} ^your_ip$
RewriteRule (.*)/$ $1/index.php

RewriteCond %{REMOTE_ADDR} !^your_ip$
RewriteRule index.php$ index.html

To ensure that only your IP can easily see index.php and everyone else might find index.html

or even:

DirectoryIndex index.html

RewriteCond %{REMOTE_ADDR} ^your\.ip\.000\.000$
RewriteRule ^index.html$ index.php