Trying to get a PC to boot off a bootable SD card that is inside an USB attached Android device First, I'd like to make myself look less a madman than I may have appeared to be.
I wanted to have a bootable USB stick with me at all times, but it's less convenient because it's an extra object and it's easier to lose, forget etc. Then I thought, I have an Android phone, and it has a micro SD in it; perfect.
Not all PC's can boot off a card reader, but the phone itself is a card reader when attached as a disk drive.

Or at least so I thought. Turns out that my netbook BIOS (tested on Asus eeePC) refuses to see it as an external harddrive; it only recognizes it as a generic USB device and doesn't offer an option to boot from it.
The device has a name "Android phone" or smth, so it seems to me that it doesn't work as a "pure" card reader, and instead still manifests itself as a phone. 

Can it be somehow overridden?
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.