How well does such a solution scale up? You need to keep the requirements low enough that it runs on mobile CPUs, but if it becomes widespread enough, doesn't it make sense for a bot farm to pick up a Bitcoin mining ASIC to grind out the hashes for them?

so far it work great. in future issues might arise, but then i can always tweak hashing algo or switch blockchain. So these ASICs, created purposely to crunch hashes for my service will be obsolete the moment i tweak it a bit. So it have to be software...

As for mobile/desktop/etc - I would expect each community to have their own main audience to which site owner can tweak `complexity` parameter. And in V2 work will be happening in background while you browsing site, so when time come to post comment - enough work already will be done. Hope this make some sense :)