@Bryce: Sorry, I did print that article even before getting the 464+. But I read that too fast I failed to notice that sentence that stated what components were required to get the 128k expansion... so I went the hard way and checked the circuit diagrams (and failed to notice R55). I've got now every component except that nasty SED9420 (I bought it on internet, and it may appear in a week or two), so I'll put R55 when I got some time and the rest of components when the SED9420 comes.
(I haven't got a uPD765, but I guess that a 8272 will work)
@redbox: The cartridge is the same for 464+ and 6128+, but I'm not using the original system cartrige. I'm using the spanish + parados cartridge. I guess that cartridge boots directly into parados, and parados keeps waiting for a FDC that does not exist. This behaviour may not be (and I guess it is not) the same as the original system cartridge. The original system cartridge is expected to work in every computer in the plus range (maybe excluding the GX4000), so I guess it will bypass the AMSDOS ROM if no FDC is available.
Unfortunately, I'm out of available EPROMS (I've got no EPROM eraser). So until I manage to erase one of them, I'm stuck with the system + parados and the diagnostic EPROMS (enough to check the repairs done).
I've done some comparing between the french original system and the patched (1.0) one, so I've managed to do a 1.0 patched spanish ROM (I did it previously with the megaflash patch for 6128 OS). Fortunately, the system ROMs for different languages are similar, so it's easy to check those patches. Maybe I'll take a look to the rest of the patch to see if it can be "advanced" to your 1.3 version.