I always wanted to enjoy this portion of the game with good/neutral character that I always play (as, perhaps, some of you too).
So, my idea is simple - to make a little mod that adds:

• a dialog entry for all your targets saying something like: "Beware, the Dark Brotherhood is after you. You must stay hidden or leave the province for some time at last".
  
• perhaps, but not necessarly: a condition that you must have some fame, or be with high disposition with that character, or in Blades faction - if not, the character won't listen to your warning

Your target will then walk away somewhere or just disappears from the game after a day.

It is a similar concept that's in Whispered Warning mod http://oblivion.nexusmods.com/mods/6189 I looked into it in Tes4Edit, but failed to recreate it.
So, I hope that maybe someone will be able to create such mod. Would be great. I'm almost sure that I'm not the only one who would gladly use it.

All the credits would go to mod maker, I could be only blamed for the idea