Using any other crypto currency for that purpose would require Ripple to juggle with multiple chains
How so? Simply attach a transaction fee payable in Bitcoin (or something else).
How does the Ripple client pay the transaction fee if it is offline from Bitcoin? If your answer is offline transaction, then how does the Ripple client knows what Bitcoin transaction outputs to use to pay the fee? And who will push the new fee transaction online? The rippled server? Then it needs to be a Bitcoin client as well.
Then comes validators: how validators know that the output of the attached Bitcoin transaction have not already been spent if they are not themselves Bitcoin client receiving the Bitcoin blockchain?