Good idea - and very close to one I've been working on in a desultory way for quite a while.
My cut on it was to make a mini-tank shaped device with RCA phono jacks in and out that could be plugged in to replace a spring tank. There are a limited number of input impedances and levels for spring tanks, and even fewer output impedances and levels. The Belton BTDR-2 digital reverb gets good reviews for sounding much like a spring reverb. I've done some minor design, adjusting input level and output levels so the interface to the amp would be the same at the ends of the reverb cables.
The only addition needed would be a source of 5V at up to 200ma. This is a modest PITA in a typical tube amp. Messing with making this from the heater supplies is possible, but frankly the simplest thing would be to wire in one of the 5V output wall warts inside the amp.
Sadly, my ability to design things is far larger and faster than my time to get prototypes built. The boards are laid out and ready to print, pending only some mechanical mounting issues. I have an amp-tech friend that has squirreled away a couple of dead small-size tanks for me to use in making a proto, but he's out of town, the sun is in my eyes, I have blisters on my fingers....
On top of that, I have just received my first run of repro Thomas Vox knobs. Chrome, knurled, very, very close to the real thing. Sigh. So many designs to do, so little time.