As a libertarian, I would argue that no one has the right to coerce either the donor or the politician to disclose their donations. They are not coercing anyone, so, by the NAP, they are fully within their right to go about their business unbothered.
To a principled libertarian, the above is all that matters. A libertarian focuses on rights, which are discerned through reason alone. The effects, as I will discuss below using a praxeological method of reasoning common among libertarians, are of no importance. To give an example: On a more popular topic, gay marriage, it matters none if the effect is, as some Christian conservatives may argue, to destroy society; it is still a right to do it, because it doesn't involve coercing others. The same is true of all actions that do not involve coercing others; it is anyone's right to be allowed to do them. That seems perverted, but if you read "Liberalism" by Mises you will see that liberty is the glue that holds society together and that the idea that any restriction of it can be helpful to society is a contradiction.
Now you say "but what about people buying off politicians"? And to that I respond: there's no incentive to buy off officials in a libertarian state. By definition, all the state does is prevent others from coercing. It is limited to only that and anything else is prohibited. So what can a bribe achieve?
Further, it seems to be within reason that, if it is important to voters, open campaign financing would arise simply from the competition of politicians. A politician can voluntarily agree to disclose a certain amount of donation information. This can be enforceable as a contract between them and each donor, for example. Then, if they lie and anyone finds out, any donor that chooses to could sue them. And this has the added benefit of allowing a sort of market to decide how much data should be revealed. If most people want more info, they won't vote for someone who doesn't disclose enough. (Sending a signal that more data should be public, for the sake of the voters.) If donors want less info exposed, they will donate to those who disclose less. (Sending a signal that less should be public, for the sake of the donors.)