I have never used a MCP23017. But from the data sheet, you seem to be able to disable the SEQOP bit in IOCON.
Then having addressed a register, you should be able to write data continually to the same register. i.e. much like the PCF8574.
Untested.
I must admit. I can't see much point in using 8-bit on an LCD. You can't read an LCD very fast anyway.
David.