AFAIK the only systems that can reliably do this including chords are guitar synths, which require special pickups that capture the vibration of each string independantly. Some guitars are made with these special pickups already integrated, so at first glance it might look like an ordinary guitar + pedal arrangement.

For example: https://www.roland.com/global/products/gc-1/

I think this would be either EHX Key9 (which actually doesn't have MIDI) https://www.youtube.com/watch?v=av4X84lChEE or Roland GR55 https://youtu.be/YqKZYUExYCs (which actually can be used for other sounds than piano), or possibly some older Roland device.

As others have said, it was probably a Roland GR-series unit connected to a guitar with a hexaphonic pickup.

If you're a computer-based musician, an excellent option is Jam Origin's Midi Guitar 2. It's a pure software solution that accepts any guitar as an input. It works polyphonically, has low latency and tracks the notes with remarkable accuracy.

https://www.jamorigin.com/

I bet it was the Roland setup. I forget the model, but it uses individual pickups for each string and some huge 9 conductor cable to go to a decoding box. I still have one somewhere. The latency was terrible, and more than one note at a time was a gamble, but it was fun for string sounds with a slow attack. Some guitars in the 90s even had the setup built in.

To go back even further, check ot the original, the VOX Guitar Organ. Lots of vids on youtube.

With more modern tech it should be possible to make quite a small decoder that fits into the body cavity, then shoots out the MIDI via Bluetooth LE. (And fix the latency issue to boot.)

Do you know if anyone has attempted such a feat?

Am embedded electronics designer, for context.

Yeah sounds like the Roland GK-3.

Expensive, finicky. But it did convert to MIDI for sound purposes. Enough so that Fender even sold a model with the Roland pickup built in.

Source: 20+ years with guitar and way too nerdy with MIDI and stuff over the years

Aside from Roland technology, Graph Tech makes the Ghost system which - depending on the model - can provide MIDI or acoustic piezo individual pickups embedded in the guitar saddle. Godin does a lot with Ghost saddles for their MIDI guitars. The Ghost system is the only MIDI pickup solution that offers an invisible look to it. Fishman's TriplePlay also offers this functionality, but it isn't a built-in interface.

As digital pedals get better I'm sure we'll eventually be able to get away without a MIDI pickup, but I'd bet overtones inside some chords could really confuse a system which is effectively trying to decode frequencies in real time to identify the fundamental. However, with that said, Digitech is getting really close with this ability in their long-running Whammy pedals to transpose entire chords as well as individual notes.

Not piano, but could it be the EHX B9? It's an organ emulator for guitars. Not really doing the full pitch/chord detection you would need for generic MIDI output (I believe it's vocoder based, but I might be wrong).

> I remember being in a guitar store and hearing a pedal that converted a guitar, chords and all, into MIDI piano samples

I remember the same experience (in like 1989).

Maybe it's the Electro Harmonix KEY9. I own this one and you can play single notes and chords which sound like an electric piano / wurlitzer etc.