First we'll have to wait and see if this can be done, but judging from the reddit posts by Dr. A. Back they are well past that point. He apparently organized a "Bitcoin mansion" with a lot of btc-devs to work out if it could be done. He also seems to be in discussion with a large number of mining-pool operators plus he already has massive funding partners. All in all I don't think it would be wise to bet against this getting implemented.
The reddit discussion also mentions that there is no limit to what type of sidechain can be pegged onto the bitcoin-main-chain even other POW/POS ones, but preferred merge-mining over inferior solutions like scrypt if you just want things like faster block-confirmations.
Closest analogy to I3 projects would be that bitcoin works like PTS, but without snapshots, so you can freely opt in or out of your stake anytime you want. The lag of getting BTC in and out of the sidechains is only required for liquidity and interchain btc-trading would probably be faster. To me this makes all alt-coins plus things like Ethereum in their current form irrelevant and Bitcoin into a giant Hydra of the crypto-space with transforming heads.
I share the concerns of Bytemaster around bitcoins mining centralization, however sidechains also offers an easy way out for bitcoin, as this essentially makes bitcoin entirely liquid and blockchain independent.
All in all I think I3 and Bitshares should add the btc-sidechains into their considerations for future (or even short term) projects. I know I3 associates itself with the virtual giants fighting knight, but even chivalrous idealists have to eat. If btc-sidechains can absorb our DACS it might be wise to be doing the absorbing ourselves and offer a more gradual and incentivized way out of bitcoins centralized mining. A bit of strategic and tactical readjustment might be advisable.