I think that adding a clause mentioning or prohibiting the appearance of a conflict of interest is a good idea, as long as it's understood what “the appearance of a conflict of interest” is. If a conflict of interest itself arises when an official is in a position to affect their interests, even if they are actually uninfluenced by them, then the appearance of a conflict of interest is when they appear to be in a position where their interests could affect their judgment.
An example of that from the United States arose when the Comptroller of the Currency, many years ago, divested all his shares in federal banks that fell under his jurisdiction, but he was accused of being in a conflict of interest because he still had shares in state banks, whose interest he could not affect in office. However, it was deemed to be an appearance of a conflict of interest because the public might not make that distinction.
It's a finer notion, the appearance of a conflict of interest, but certainly I think that officials have an obligation to avoid not simply a conflict of interest, but the appearance of such. To put it another way, their conduct should pass muster even with the most casual observer, meaning somebody who really doesn't pay great attention to the details and might misunderstand them. His obligation is still to make sure they don't draw that conclusion.