Running Windows 7 from SD card on a Macbook Air
One question posted at Ask Different deals with running Windows from a external drive. While the questions deals with Windows 10, the answer seems to indicate a BIOS boot method is used. Therefore, the answer might be adapted to work with Windows 7. Below is a link to this question.

Is it possible to use Boot Camp with Windows 10 from an external HDD?

In your case, the problem really is not whether you can get the software to work, but rather the hardware media you are choosing. For a while now, Microsoft has had the ability to run Windows from a flash drive. This is referred to as Windows To Go. If you read the link, you will realize this requires the use of special flash drives design to endure the large amount of reads/writes required by the Windows operating system. A normal SD card probably would not survive long under such conditions.