Let $g$ be a generator of a multiplicative group $G$ of order $q$, $x$ be a private key, and $h=g^x$ be a public key of an exponential ElGamal cryptosystem.
Given a ciphertext $c$ produced as the ElGamal encryption of a given plaintext $m$, that is $Enc(m,h,r)=(g^r, g^m h^r)=(R,S)=c$, it is obvious that this ciphertext can be decrypted by using the private key $x$. So $Dec(c,x)=\log_g \frac{S}{R^x}=m.$
However, it seems also quite clear to me that another way of decrypting it without being in possession of the private key is by knowing the random value $r$, so $\log_g \frac{S}{h^r}=m$.
Let's assume we have a protocol in which Alice first publishes some encrypted message, and after a while, she wants to reveal it by just revealing what the random value $r$ was. This may remind a Pedersen commitment scheme, with the difference that when opening the encryption, I would like Alice to reveal only $r$, and not both $r$ and $m$.
Does this make any sense? How certain can Bob be that the value $r$ that Alice just revealed, is the one she actually used to generate $c$, and she didn't make it up?