What encoding or format is the private key in Dumpprivkey?
The format is base58check, with 0x80 as version byte (for Bitcoin mainnet, at least), and an optional suffix 0x01 byte to indicate that the corresponding public key is compressed. So combined:


base58(0x80 | 32-byte big-endian secret parameter | checksum) for uncompressed public keys
base58(0x80 | 32-byte big-endian secret parameter | 0x01 | checksum) for compressed public keys