How to get the private key in hex from the output of dumpprivkey
OK, I managed to do it with the excellent bitcoin-tool by matja
https://github.com/matja/bitcoin-tool

git clone https://github.com/matja/bitcoin-tool.git

cd bitcoin-tool

make test

./bitcoin-tool --input-type private-key-wif --input-format base58check --output-type private-key --output-format hex --network bitcoin  --input "KzRVLwCuCi2tH2PpSNZnwUqPwg6gaqXAM2LWobUurzAsNdC5JesE"

which gave me 
5f92519eef7ec4e37cbcd25f9ba4a82e5acf9a8cd274d4d3aa3aa727fd7b6969

as a result.