Got there in the end. Bottom of Section 2.6.1 of spra911c:
'To avoid corruption of these memory locations, the sections contained in the boot table should not contain any destinations lower than word address 0090h (byte address 0120h).'
Mis-reading word addresses as byte addresses, and not realising that the bootloader wrote and interpreted the lower locations of RAM to its own ends ... I hope these findings are useful to somebody else, because they utterly did my head in.
I wonder if it might be worth somebody at TI putting a note to this effect in the linker script files? How many hundreds of pages have I read to chance upon this troublesome paragraph?!