In-game you can press '~' and enter the following string after the 'Omen' prompt to get full invulnerability:

mdkGobSetDamageFilter(mdkGetPlayerGob(),0)

Looks like the programming language? Yeah - this is trick for the internal game scripting system. But please note that this code must be re-entered periodically because the game doesn't store previous mode while map block change (block changing can be determined if your HDD reads something heavy and the game slowdowns for a while, or just see what you now is not invlunerable and your health value is decreasing). Hopefully you do not have to re-enter this long code - you can just bring console ('~' key) and then press Up-arrow key to get previous command form history.