Can you today use bitcoind with the private keys outside of the wallet without development complexity?
The only way to assure 100% uptime is to distribute your cluster and for that you need to handle private key distribution outside of the closed wallet.dat file.
What would the way to do this with the raw transactions API look like?
This isn't what you asked, but it reflects what I recently discovered. You might find it of interest. I was able to run Multibit-HD and access private keys on a Trezor on the same computer as running a full bitcoin node. I kept the multibit-HD client isolated from the bitcoin network by using a firewall to block it. The client was forced to go through my trusted full node, preserving the privacy benefits of full nodes.
Multibit HD's peer discovery software as shipped finds the bitcoin core via localhost. Unfortunately, at present Multibit HD lacks the peer configuration flexibility presently in bitcoin core. If it had it, it would be possible for one or more appropriately configured clients to access a set of trusted full nodes that would comprise a "cluster".
