The theory is this:
Any electronic fuel injection engine has to pulse the fuel injectors to inject fuel. A fuel injector flows some amount of fuel per minute (it's usually stated in something like 850cc/min). So, you can theoretically hook to one injector, measure the total amount of time the injector is open, and then multiply by the number of injectors you have.
As someone who just put new injectors on their car last week and is still trying to get it running right, I can tell you the reality is not the same as the theory. We can measure the electrical signal going to the injector, but the injector is an electro-mechanical device, and nothing mechanical happens instantly. Thus, there is a latency (delay) from when you ask the injector to open with electricity and when it actually does. More than that, this latency is highly dependent on fuel pressure and battery voltage.
The latency can be much longer than the amount of time you want the injector open. Latentices are generally in the 1-4ms range, but at idle, you may only want the injector open for .5ms. Thus, the computer may apply electricity for 2.5ms because it knows it won't get fuel for 2ms. So while you actually only got 0.5ms of fuel, SkyView would think you got 2.5ms (5x as much!)
Of course, now your EMS needs to have all this data to know to ignore the latency, and it needs a table of voltage and fuel pressure against latency. I'm guessing very few customers have this table available.
So, while it's all possible, it's not as easy as it sounds (or we can do it the easy way like other companies do and end up with big errors). Fuel flow senders are proven, reliable, accurate, and not that expensive. Given the number of EFI engines our customers appear to have, we're talking about lots and lots of engineering time in order to save a few customers $200 on a fuel flow sender and that doesn't pencil out.
We're happy to hear there is more demand than we thought and will consider doing this. There is also the option that you can get your ECU manufacture to change their output from PWM to frequency (it's "just software" for them as much as it is for us). Someone could also design a pretty simple PWM to frequency converter that would allow our standard system to work.