Thanks for the comprehensive reply.
To clarify my answer: I don't expect the shield to make the Galileo 100% compatible with the Arduino. This makes no real sense. For the price of the Galileo and your shield, you might get 15 or more Arduinos.
Instead I hope, the shield can make the migration easier.
Let me explain more in detail.
The function Arduino - PulseIn isn't implemented on the Galileo. But this function is essential for using sensors like bare ultrasound sensors. It did some research on how to implement it, like you can directly use the sysfs devices to emulate digitalWrite or analogWrite. No way. To make it even harder, it might be neccessary to modify the kernel driver to implement such a function, and that still might not deliever reliable results.
But with your shield, it would be no big deal.
Now let's see this from the view of Joe McMaker, your friendly neighbor and student who likes to do some tinker projects during the weekend. He have some experience with electronics, but often relies on Google to find a suitable Arduino sketch, that matches his needs with some modifing. And to be realistic, today, for each existing Linux/C/Python/whatever program to control hardware, there are 100 Arduino sketches doing the same job.
He will understand, if you explain him, pulseIn() and other functions will not or imperfect work on the Galileo, but he will also not have the neccessary time learn how to deal with FPGA. It would be nice to have a well documented library "fpga.h" and a preconfigurated FPGA, that covers such funtion like pulseIn(). So he could replace the original calls of pulseIn() with the fpga_pulseIn().
Later, he might be interested in playing with the FPGA directly, but he shouldn't force to spend time on a specific hardware that ist not directly related to his project at this point.
In the answer to BillS, you expressed your thoughts about a bus-based connection between the sensors or not. I recommend you to take a look at Tinkerforge , this project is not far away from your idea and they are using a bus.