My SMBus controller (SMBus PIIX4) is also lacking the I2C Block Read functionality, but I was able to get the check string using byte read:
$ i2cset -y 1 0x50 0x0b 0x04
$ i2cdump -y -r 0x89-0x8b 1 0x50
No size specified (using byte-data access)
0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
80: 4b 46 35 KF5
Maybe it's going to be better to implement the check fallback like this, for better compatibility?