PWM the fans at 1 Hz! Use a minimum pulse width of say 100 ms. When staring up you can give a couple of seconds of 100% duty cycle to make sure they start spinning. This has worked on every two wire fan I have tried. Of course the tach output wont work very well with many fans, but it does with a lot of them.
My PWM fan controller PID code n Lady Heather can control the temperature of a GPSDO to millidegree levels... micro-degree long term (actual absolute accuracy dependent upon the temp sensor)
That's not the point of this project.
The intent was to use the PWM control already present on many PC/Server motherboards to control high-current non-PWM fans. In my case, a rackable systems NAS server (which I'll use for other things too) that had 4 high current fans on proprietary hot swap connectors going to a dummy boards that took a standard "molex" plug's 12V and put it on 6 standard fan ports. The Tach was wired up (but not used by the dummy board) but the fans are not PWM. I have a buddy with a similar situation (old dual-motherboard monster case, far newer motherboards to drop in), hence the idea.
To do what you're suggesting would require the addition of a microcontroller with few exisiting components removed.
As for fan spinup, Motherboards already handle this. Since it's a pulled up signal (on the fan/PWM amp side), no signal=full speed. Likewise, PC motheboards from my experience run the fans at full blast during startup as normal procedure either intentionally (to spin the fans up) or because the low level controls haven't started yet.
Using the onboard PWM is desired as it both is an already controlled (sometimes with tachometer feedback) signal source that can also be controlled from software on some platforms. In addition, for non-PWM systems there are already 3rd party PWM controllers for computer fans out there, so one could combine this with that (or do both and use one channel for each) making this a partially universal solution.
I see no need to go through a bunch of trouble to convert/interpret a PWM signal to a lower frequency when the hardware to directly use and drive with the higher frequency exists and isn't cost prohibitive.
Why reinvent the wheel?