I don't think Electrum has a limit to the length of an extension, but some hardware wallets do. Trezor has a limit of about 50 characters, so if you want to add an extension to a Bip39 seed phrase and want it compatible with hardware wallets, that'll be a limiting factor.
You should never use a seed generated through anything other than the HW wallet itself if you're primarily using it on a hardware wallet. That is a non-issue as you probably wouldn't want to expose your seed (and passphrase) to an Electrum instance while using Trezor.