You cannot customize the following HTTP errors: 400, 403.9, 411, 414, 500, 500.11, 500.14, 500.15, 501, 503, and 505.

I want to have some user friendly error message when app pool goes down/deployments or iis is stopped on SharePoint site. I already know app offline htm technique which does not fit our needs.

This says its not possible. Can I directly edit these htm files under custerror folder in IIS?

MCTS Sharepoint 2010, MCAD dotnet, MCPDEA, SharePoint Lead