Plausible? What does that mean, exactly?

What subjective probability would you assign to it?

Not every substance can perform every sub-part role in a consciousness producing computation, so there's a limit to "independence". Insofar as it means an entity comprised entirely of non-biological parts can be conscious, which is the usual point of contention, a conscious system made up of a normal computer plus mechanical parts obviously shows that, so I'm not sure what you mean.

I don't know what the "usual" point of contention is, but this isn't the one I'm taking a position in opposition to Bostrom on. Look again at my original post and how Bostrom defined substrate-independence and how I paraphrased it. Both Bostrom's definition and mine mean that xkcd's desert and certain Giant Look-Up Tables are conscious.