Another potential candidate data-chain would be STEEM. It is designed to store large chunks of consensus-meaningless data (hypothetically posts, but it is culturally accepted to store anything as long as one has the resource credits.) It also has “free” transactions under a sort of fractional reserve system. Hypothetically, as a DPoS chain it will have an easily verified light client system. Or so I thought.
Unfortunately, as far as I could tell, there is no simple way to compute the current witnesses except by manually counting votes. This is not a problem for a full node, but obviously impracticable for Ethereum. As I wrote this it occurred to me that the witnesses could fork STEEM to include this information, but that would ultimately be self referential–“Hey, we’re totally the legit witnesses to this block, the block said so!”
In, re: the general idea, how much of this can be done with off-the-shelf parts? I haven’t dived into Truebit yet, but if it can apparently read from IPFS or onchain (ETH) storage, then it can be rewired to work with BCH. (For that matter, if you didn’t mind a little subjectivity, you could create some STEEM-to-IPFS gateway and just hope it all works.)