You simply cannot. The HD seed is used to derive the master private key, not the other way around. This derivation uses a hash function so it only works in one direction: HD seed to master private key. Given the master private key, you cannot get the HD seed used to produce it.
BIP 39 mnemonics are used to create a HD seed which is then used to create the ...
Is there a neat way to pull in all funds that derive from a "master key". Or, have I misunderstood deterministic benefits - am I trying achieve a non existent feature?
If you import your master key (which may be in the form of a mnemonic or an xprv...), any BIP44 compliant wallet should be able to locate all the funds, provided you followed BIP44 when ...
You need to use the decode flag -d:
printf "5HueCGU8rMjxEXxiPuD5BDku4MkFqeZyd4dZ1jvhTVqvbTLvyTJ" | base58 -c -d | xxd -p
To have xxd output all on one line, give it a large column number -c flag:
$ printf "5HueCGU8rMjxEXxiPuD5BDku4MkFqeZyd4dZ1jvhTVqvbTLvyTJ" | base58 -c -d | xxd -p -c ...