Perhaps a forced multiclass? Cleric/rogue, with the cleric class level never being allowed to be higher than the rogue class. The rogue gets the sneaky-skills you seem to be after, and by switching a couple of the rogue core skills (ie the ones you want to be exclusively for rogues) for knowledge (religion), spellcraft, and concentration. First level could be approximated using the apprentice-level multiclass option in the DMG. If you wanted to take away the other rogue powers like evasion or sneak attack, you could give 'em improved hit point dice for the rogue levels, and maybe allow effective caster and turn undead level to be the sum of cleric and rogue levels.
Some work, I suppose, but has the advantage of not buying any new books, and using rules players are already mostly familiar with.