Trying to get a PC to boot off a bootable SD card that is inside an USB attached Android device
No, it can't be done with an Android phone due to the driver installation requirements that Android has before it will connect the SD card to the PC as removable storage. The Android OS requires basic (charging/connection) drivers to be installed before it will act as removable storage.

Because these drivers can't be installed in POST, the phone will not offer the SD card as a storage medium.

There might be a way to do it in Android, but I suggest you flag your question for moderator attention and request migration to Android SE if you want a possible Android solution.