Which block explorer indexes the keys used in mutisig scripts? Consider transaction 0846e91bc3afdf2275db14bcbb10e19f141069d77f17dae6bcbd907fd28f4747, which takes money out of the script address 3PJQGGnBQr8DJBdPYowd6Mp2kSWvFFo7Gk. As we can see in the input scripts, this script address references script 51210261afd7a2796e9bb83896089293f11edd2987ebbd67ed2075f5e45b7c4bb7edcb51ae, which, according to the decodescript command, is

    "asm": "1 0261afd7a2796e9bb83896089293f11edd2987ebbd67ed2075f5e45b7c4bb7edcb 1 OP_CHECKMULTISIG",
    "reqSigs": 1,
    "type": "multisig",
    "addresses": [
        "1xW3APNVG6Ro1HY8hysmnKq8mvgVk6oPU"
    ],
    "p2sh": "3PJQGGnBQr8DJBdPYowd6Mp2kSWvFFo7Gk"


So this payment was made to anyone who has access to the key with the address 1xW3APNVG6Ro1HY8hysmnKq8mvgVk6oPU.

But none of the common block explorer websites seem to index this; when I ask them about the address 1xW3APNVG6Ro1HY8hysmnKq8mvgVk6oPU, they list no transactions.

Is there a block explorer that maintains a mapping from key address to the script addresses the key is mentioned in?
You must not see the address in the blockchain as a receiving address. It is a P2SH tx, which means, that the spending condition is sha256’d. This is the type 3address above. The 1-of-1 multisig condition inside defines, that anyone who has the private key to the “1xW3APN...” address can spend this tx, which happened on 2014-07-28 to the target address “1UxOYSY...” 
I highly recommend reading Andreas’ book mastering bitcoin (online available), which explains in nice detail the reasons and insights of those multisig adddresses. 

Blockexplorer blockchain.info obviously doesn’t show the decoded script, cause it is not an address used to receive funds.