Yes, you may be right that the T1 looks for a bootable sd card.
And the big question is - what is considered "bootable"? Is it as easy as dumping internal flash device contents including boot loader, kernel & friends 1:1 onto external sd card? I'm not sure.... Might as well be that the T1 just _initializes_ external SD, but doesn't consider booting from it. But on the other hand there must be a way for Sony to recover devices....
Also, the encryption key within the first 25MB might be an obstacle. Find info on it here:
https://www.mobileread.com/forums/sho...d.php?t=166789
https://www.mobileread.com/forums/sho...d.php?t=166871
At freescale you might find valuable info, too: They have Linux images, Android images, SDK and Docs on the iMX platform which the T1 is based on:
http://www.freescale.com/imx508