Syntax and phonology are intimately related in mysterious ways. Let's start with morphonology.
Einheimische zu gewinnen, […] habe er zehn Jahre lang versucht - doch das sei unmöglich, meint Chambers.
Es is parasitic in High German varieties. See the absurd Es sind dies ... construction for example. In my humble opinion it is exceedingly likely that e- reflects *h1 or was geminated to maintain phonotacticts.
Cp. ist < PIE *h₁ésti; sind < PIE *h₁sénti (Ringe 2006, From Indo-European to Proto-Germanic, pp. 30, 195, 262). See similarly s'regnet "es regnet" The stark difference being zero onset vs. glottal stop. The vowel follows because prosodic syllables cannot be entirely voiceless.
The root is *h₁és- ~ *h₁s-; or "*es-, schwundstufig *s-" in older works (Pfeifer/dwds.de)
As a referential pronoun, both das and es are available.
Einheimische zu gewinnen, […] habe er zehn Jahre lang versucht - doch (das, es) sei unmöglich, meint Chambers.
Once es is interpreted as dummy pronoun, it creeps in anywhere, the reference has to move to another place. This explains fairly little about the actual question of "left-dislocation" if course. The diachrony of it must be quite complex. The joke is, you can remove the dummy pronoun: "? Einheimische zu gewinnen, das habe er zehn Jahre lang versucht - doch [… is] unmöglich, meint Chambers."
This is prodrop and quite usual in spoken informal registers. It is permissable at the very least under the fast connected slurred speech rule, i.e. assimilated *abbas unmöglich < aber es ist, intuitively speaking. aber is required for its final vowel, which doch does not have, which is why another vowel may be added to it as I have argued.
In result, transposition works out: "? Einheimische zu gewinnen ist unmöglich. Das, meint chambers, habe er zehn Jahre lang versucht," The fact is Chambers only claims to have tried, so the head of the clause which vb. meinen has to connect to has to be das, irregardless where it stands, unless the intermittent space is far too long. This too implies fast speech, simply because the time space is shorter in that event, leaving room for even longer arguments before the stack overflows.
Since the derivation of the modern auxiliary verb paradigm is much more complex, I have not found sei per-se in the mentioned material. Chances are there are bigger fish to fry.