Hmm - plug-in may still be relevant for sony PRS-T1 ?, at least until PRS+ is confirmed to work on the new device.
and for folk like me who just want to add some book cover images, without all the hassle & stress of having to reflash firmware & then having to master a different UI. I just re-googled the PRS+ install instructions for a PRS-650 with latest Sony firmware. & it still looks daunting. e.g. I'd apparently need to learn how to "russify"- whatever that means