And here is the proof. It can be done. I've surprised myself at how easy it was to do this upgrade, but you will need some familiarity with a soldering iron and the case modifications are not for the faint of heart.
You may be able to find one elsewhere and please let me know and I'll put the information here, but the link above seemed to be the best price from what seemed to be one of the few legitimate looking sites, so I went with that. Be wary of postage and import duty if ordering from outside the U.S. though.
Anyhow, I struggled with the explanation in the original how-to, but took the plunge anyhow.
Firstly you need to make room for the PA2067U module inside the casing.
WARNING: please do not attempt this if you are not familiar with the basics of electronics, soldering, and how to use a Dremmel (or similar) and remember to use anti-static measures.
To get the memory module bay, remove the battery (and the power), and carefully lift the strip at the top of the keyboard from the right-hand side. Once fully removed, you should be able to lift the keyboard up and away from the bottom half of the machine.
You should be able to see the memory slot in the bottom left area where the keyboard was. If you had the official memory upgrade for the Libretto, you could just slot it right in, put everything back together and boot-up with a full 64Mb RAM in your machine. Unfortunately, the PA2067U is not really intended for the Libretto 110CT, but will work with the correct mods.
To do this, you will need to take the machine apart further. Remove the keyboard completely by removing the two silver screws at each end of the keyboard recess and then carefully push open the securing clips at each end of the ribbon cable connector, so that you can slide the keyboard ribbon cable out and then remove the keyboard completely.
You will also need to remove the thin metal plate that is under the keyboard. Under are three connectors which must be disconnected: on the right side is a connector for the screen, another one at the top left (I think I remember this is the microphone connector or something), and one near the bottom which is for the CMOS battery.
Turn the unit over and remove all the screws from the underside. To be on the safe side, remove the hard disk from the bay on the left side of the machine and place safely to one side.
Once this is done you should be able to gently prise the screen and the top half of the main unit apart from the lower part of the main unit and motherboard.
Now the tricky part: there is a silver strip along the bottom edge of the keyboard area which is in the way of our new memory module, and the only way it will fit is if we remove most of it. By trying the module for size you will easily see how much needs to be removed. I snapped the silver strip away with some long-nose pliers and smoothed it off with the Dremmel tool, but I can't emphasise how carefully you need to do this bit as you could really ruin the casing of your Libretto if you are a bit too heavy handed with this. See the picture below and compare with your own to see where I have removed the silver plastic from mine.
You also need to remove some of the plastic over the screw hole just to the bottom right of the memory slot. Not only that, but the connector for the CMOS battery gets in the way of the memory too. This is quite easy to snap off from the motherboard, but this will mean that you'll have to solder the wires from the CMOS battery directly to the motherboard as you can see I have done above. Some insulation tape will be needed over this, as the module will sit directly on top of this.
With this all done, and the plastic halves of the lower half of the unit put back together the memory module should snap into place nicely. If not, then you may have to trim away a bit more of the casing until it fits.
Now I won't pretend to understand why, but to get the Libretto to recognise the full 64MB (and not just half) you need to select the middle jumper on the upper left side of the module. I did this by putting a small blob of solder between the two jumper contacts as shown in my photo below.
Now put everything back together! Don't forget to reconnect the connectors under the keyboard.
Once you have your Libretto all in one piece, hold your breath, boot-up, and check your system properties, and if all is well the memory should report 96Mb instead of 32Mb!
Whew!
