That may not even be slow enough for the 128 KHz WDT oscillator. But, good news, you're using Arduino as ISP. Remember the first step of using it, where you upload Arduino as ISP onto the board you're using as programmer? You have the source code for the programming tool right there!
Open Arduino as ISP, save a copy of it, and find the line where they set the SPI clock. Line 53 looks promising:
#define SPI_CLOCK (1000000/6)
Changing to
#define SPI_CLOCK (100000/6)
ought to do the tric, upload modified arduino as ISP to the arduino being used as p[rogrammer, burn bootloader to target using correct settings.
Edit: Haaah! a few lines before that they actually gave example. They did 128000/6 - but I said 100000/6; since the WDT oscillator is not carefully calibrated, and we don't really careabout it being 28% faster, but we do care about itfailing to program the part because you got unluicky and have an unusually slow WDT oscillator (they are not very carefully calibrated - where it's given, the spec is like +/- 30%)
That's how I'd probably undo something like that.