can you not do something more naive:

• have flumeview plugins loaded in-order so that a new module, say ssb-predecrypted when plugged in first gets passed messages first.
• then when a plugin comes across an encryped message, you send a request to ssb-decrypted to handle it, which has been building a plain text cache

e.g.

server
  .use(require('ssb-predecrypted'))
  .use(require('ssb-backlinks'))

//// in ssb-backlinks

  if (typeof msg.value.content === 'string') {
    msg.value.content = sbot.predecrypted.getContent(msg.key)
  }

you could even pass every message over and put that boolean logic inside ssb-predecrypted