A French company called Anasounds have just released a genuine spring reverb pedal/tank combination. And it looks awesome, but it's not cheap. Doesn't look like there's a schematic available yet, either. Would be good to try when there is, though!
I agree, that's probably the best option. But they don't sell PCBs on their own, only ones with components, and those retail at £57 each. And I'd be after two, for stereo reverb, so at £114 plus two reverb tanks it gets a bit pricey and I might just leave it for a while and stick with the digital one I've got!