I suspect there are dedicated PWM controllers that will do what you want. If not, it would seem easy to do with a microcontroller. Assume a PIC operating at 16 MHz (common 8-bit ones easily go to 32 MHz). The instruction time is 250 ns (16 MHz/4 = 4 MHz system clock = 250 ns per clock cycle (Tcy)). Using 1-cycle instructions should give a 250 ns pulse then a delay. Here is some off the cuff untested code. Assume the output bit you want to toggle is PORTA,0 (or LATA,0 for those with output latches.
Start
bcf PORTA,0
movlw 0x01
Pulse
xorwf PORTA,f ;PORTA = uuuuuuu1 where u = unchanged
xorwf PORTA,f ;PORTA = uuuuuuu0 (elapsed time = 1 Tcy)
Delay
; add whatever delay you need for low
goto Start+1
end
Of course, how fast it really toggles will depend on how much capacitance it is feeding. One might use bcf/bsf pairs, but bit instructions can cause problems known a RMW (read-modify-write) errors. Using output latches cures that. I would recommend an enhanced mid-range or later chip with output latches. The 10F320/322 are available as a 6-pin device in an SOT-23 package or 8-pin DFN package.