See HowOptibootWorks · Optiboot/optiboot Wiki · GitHub
huh. The minimal interface to optiboot might involve sending a series of STK_LOAD_ADDRESS commands each followed by a STK_PROG_PAGE command, carefully timed so that they complete before the next one is sent (or you can wait for the responses; hard to say which is easier.) The other stk500v1 commands implemented by optiboot look like they're all to keep the host-side happy, rather than being important to optiboot itself.