The discussion unfolded there, and it refined the topic a lot. So let me do some pasting (first level quote is mine).

(tl;dr: self-governed inter-community fund transfer system, capitalism- and state-free.)

I see two usabilities of altcoin in current (and mid-term) solarpunk context:
Proof-of-useful-work for primary circulation means that coins can be created in exchange for resources (i. e. storage and processing power) made available to SolarPunk initiatives.
Secondary circulation makes it possible to transfer funds between distant communities, without any need for a deep trust or even knowledge. The chief matter is to avoid double-spending, of course.

This way or another, whatever is the source of the currency, we always need to address the risk of double spending and forgery. In absence of a commonly trusted authority (bank or government), public trust and control can be exercised through an openly available, tamper-proof and distributed ledger. Now, blockchain is one way to implement it. Can we imagine something else?

As for the scope of it, I see it as a bridge solution for the stage where we still have to interact with capitalist realm, but can already start building our own (post-capitalist, solarpunk, anarchist - whatever the name) real economy, so we need an inter-community exchange token. There should (nor will) not be a uniform solution for every community involved, so we need something to use as a mediating value carrier.

As such, it may retire at some point in favour of something better suited to the new world needs, or survive - like gold today - in mostly symbolic role.

Blockchain is just another name for a public (distributed/shared) ledger. Due to scaling and privacy issues of that, this method is inherently unsuited for direct payment use, but it could be useful for an inter-bank exchange market of local currencies.

…which is ideally / eventually the usecase for the coin I am thinking about.