Previously, there was an idea to break out the front panel metronome controls to an outboard keypad & display.  

https://hackaday.io/project/80525/log/175545-outboard-front-panel-standing-up

A more practical idea is to pass the headphone output into an outboard box which mixes an outboard metronome with the headphones.  It would have preset tempos & a display.  It wouldn't put the metronome sound in any recordings.  This of course, could be combined with the Wifi recording idea

https://hackaday.io/project/162680/log/181769-wifi-audio-from-the-cp33

It would now be a raspberry pi 4 outside the keyboard, with the existing board of 10 years staying inside the keyboard to capture the I2S signal & send it over USB.  The pi would have a standard web interface on a phone.  It would add metronome sounds, reverb, dynamic range compression, recording, & VU meters.  Recent experience with the ALSA mmap mode showed the latency of audio processing in Linux can be imperceptible if you just don't use any hyped libraries.  

Helas, the reverb & compression could never be as good in realtime as it is in Cinelerra.  That uses long windowed FFT's & readahead buffers.   It would have to use really short FFT's or FIR filters.