[mswift42]

I don't think there are any.

However, you easily can install alternative readers on Kobo ereaders, see here for a list of some hacks:

https://www.mobileread.com/forums/showthread.php?t=295612

[PyroLagus]

There's the BQ Cervantes, which runs FOSS software, but Kobo's open source community seems much more active.

https://github.com/bq/cervantes

https://blog.bq.com/es/bq-ereaders-developers-program/