I've made two extensions to the QMK steno code. I haven't updated this in ages so for the source code openstenoproject/qmk may be a better place: sammdot was collecting a bunch of different hobbyist mods to the steno firmware. But my source tree is also available.

One sends the chord when you release its first key, allowing you to hold down common keys in between chords. So you can (for instance) fingerspell by holding down * and tapping the letters with the left hand. Add #define STENO_1UP to your config.h to enable this feature.

The other is auto-repeat: double-tap and hold a chord, and it'll repeat (currently) 4x as fast as your double-tap (currently capped at 50 chords per second). You can also repeat sequences up to (currently) 8 chords long. Write the sequence, then press and hold the first chord again. For instance, write SUD/-PB/HREU, then hold SUD to repeat "suddenly". For multi-chord outlines you have to write the chords at an even pace: if you hesitate or speed up too much it'll break the sequence. Add #define STENO_REPEAT to your config.h to enable this feature.

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I have now built firmware for all four QMK-based hobbyist steno boards. The EcoSteno firmware is still untested but the others seem to work. The TinyMod doesn't use QMK, so porting it to that would be a whole other project.

Georgi

This is my daily driver, so I'm using the out-1up-repeat variant, and can confirm that it works. Not sure I've tried all seven (!) other variants, but they should be fine.

These should be basically the same version as the 1.1 "ClayM" firmware from the gBoards page.

The Georgi has two configurations for the thumb home position between the vowel keys: thumbs spread out or thumbs tucked in.

SOFT/HRUF Splitography

Aerick checked the 1up-repeat version briefly and said it seemed to work.

The Uni

Peter tried out the repeat (and helped me squash a bug), so these seem to work now.

EcoSteno

These built without errors but haven't been tested.

Multisteno

Tested all 8 variants and are all working as intended.

Like the Georgi, the Multisteno has two configurations for the thumb home position between the vowel keys: thumbs spread out or thumbs tucked in.

TinyMod

The TinyMod has custom firmware that is not based on QMK: I may port my extensions over at some point, but don't hold your breath...

I do own a StenoMod hinge, and I originally wrote these extensions as a full custom firmware for that. So if you own a StenoMod (they've been discontinued for several years), grab the Arduino IDE and tell it to build/upload the linked .ino file.