You don't need a hardware wallet to generate a BIP39 seed phrase, any small application or any of the dozens of HD wallets can do that. You can even create one with flipping a coin 128 times and converting the stream of bits to a set of BIP39 words. Then with any basic cryptography library (like OpenSSL) you can generate an address with less than 50 lines of code; or the alternative is obviously to use a bitcoin wallet, just pointing out that it is a very trivial process.