Try dumping it as a signetics 82s135, it is an equivalent part to the MMI6309; this is an MMI6308 part which is open collector (vs MMI6309 which is tri-state), but as long as there's some sort of pull up resistors on the 8 output bits (and any sane device programmer should already have those), it should dump properly.
Thanks Lord Nightmare for the tip.
That got me to a big list of equivalent parts. I must have a really janky programmer. There's no Signetics 82S135 (or the related 82S114N). Nor National 74S471, nor TI 28L22, nor TI 18L22, nor AMD 27S23, nor Fujitsu 7118, nor any of the equivalents for the 512x8 PROMs that different only on the chip select.
Maybe I need a better programmer/reader?