Um, did you read the question and answer at the bottom of the auction?
There was a daughter card, although I've never seen one. Back around 1995 a fellow offered to sell me three of them for $15 each, but insisted on a money order. I was willing to pay in advance and didn't want to bother with the hassle of a money order, but that wasn't good enough and I never heard from the guy again. I still gently kick myself about that sometimes.
The cable from the daughter card exited through the right-hand lower vent and terminated in a Hirose DX style connector (looks like a mini-centronics, but it's not) which is what the Laptop used to connect their external floppy and SCSI adapter.
I'm not sure how the daughter card connected inside the Mac. It was not a simple plug-in job though. There were additional connections.
However, the Outbound also supported a SCSI docking mode (forget what Apple called it on the PB1xx series) so you could connect your Outbound to a desktop Mac via SCSI cable (if you had the SCSI adapter) and put the Outbound in a mode where it would behave like an external hard drive.
But the docked-to-Plus would be very cool. According to the literature, both screens were usable when docked and all the peripherals.
Also, the NY Times article has it a little wrong. The keyboard did have an IR interface, but there was also a cable provided. So there were three connection modes for the keyboard. IR with the keyboard attached to the Laptop (there's a latch to hold it together). IR with the keyboard detached. Cabled with the keyboard detached.
The keyboard has its own battery and when that goes flat it isn't possible to turn the Laptop on. I wonder how many of these may have been tossed as "broken" because of a dead keyboard battery.