Use the CB1 input as a strobe from an otherwise unused output on the AVR, so AVR decodes keypress (as an 8-bit ASCII, encode the keys like arrows, pgUp, etc. how you like) puts the 8-bit data on the output port, toggles CB1...

The 6502 gets an interrupt, reads the data, sets CB2 - connected to another spare pin which means the AVR then knows it's been read, so can present the next keypress.