Is it possible to adjust the max password length, even if it means customizing your own firmware?

Subject says it all. I saw a post about making all the fields dynamic, which I would assume means that a shorter name/login could be used in able to lengthen the password field.
If this is the case, is there a GitHub issue which can be tracked?

I believe you want these ones: #35 (Pro) and #38 (Storage).
As for customizing firmware - yes, it is possible. I do not remember how it is done on firmware side (that is how easy it could be changed). Just search for PWS string in the code. You would need surely to update the libnitrokey too with new fields size (should not be complex) and compile Nitrokey App by hand.

There is one constraint to keep this modification easy: sizes sum of all fields cannot be bigger than 50+ bytes (total packet size is limited to 64 bytes due to USB 1.1 compatibility and probably due to device’s memory limit). It is possible to divide PWS update command to 3 parts (thus to 3 commands; allowing to 50+ per PWS field - name, login, password) but still there is a total memory limit designed to use with PWS (AFAIR 2*512 bytes). The best idea for now is to combine memory from PWS and OTP (another 512 bytes+) and use it without strict partition.

Edit: Assuming OTP secrets would be encrypted and user would not like to use OTP without unlocking device with PIN (like HOTP special keys).