Not practical, if you insist want portable device which is open-source (at least on software level), you could always bought linux smartphone (such as PinePhone or Librem 5) or use custom android ROM (such as GrapheneOS)
The only thing that you can do is to save the private keys in the notepad.
At least according to this news (
https://www.techrepublic.com/article/pgp-mobile-high-security-for-palm-os-devices/), there's PGP application for Palm OS where you can encrypt and decrypt stuff.
But i don't know if you still can install it and how reliable the algorithm from early 2000 these days.
PGP Inc. doesn't sell or even give away free licenses of PGP for Palm OS today. It's also next to impossible to find binaries of PGP for Palm OS back in the day. I think that PGP for Palm OS was used mainly by journalists with their AlphaSmart Dana digital typewriters (that run Palm OS) to encrypt messages before sending them to the central office, so maybe you can ask one if you know one who used this device.
There is a password manager called Keyring for Palm OS:
http://gnukeyring.sourceforge.net/index.html, but the webpage and the app has remained nearly unchanged since 2003.
Also, the website states:
Secure triple-DES encryption using a 112-bit key derived from the password
What? Triple-DES is secure? Do you know that NIST has deprecated 3DES in 2017?
or whether you will need to implement these yourself (ie. ECDSA, AES256, RIPEMD160 etc)
And that's how you lost your Bitcoin, either by wrong implementation which leads to invalid public key/address or weak entropy source for private key generation.
You can program PalmOS at least in C , C++ and assembler, but I guess that any programming language is able to be used given enough work.
Also, libraries written in C and C++ already exist, so porting them to the Palm OS won't be a huge problem. There is MathLib.prc shared Palm OS math library ,which itself is based on the Sun Microsystems' libm:
https://github.com/fidian/MathLibMaybe we can take the existing code for generating secure cold wallets, including appropriate libraries and port them to Palm OS. The only hard work that remains is programming an user interface to allow usage of the program and additional security features such as locking access to the rest of the OS without explicit user's consent.