While this is a bit different than what you are asking for, there is an instance focussed on archiving subreddits lemmit online faq post. That said, I like your idea, because it also puts Lemmy out there. So that maybe more will get familiar with it. And asking a poster is thay are fine with it can never hurt.
They had that plan over here. But they killed it just before the war against ad block started (it was around half of the price of premium)