Finally found the time to get this done, see here Pogoplug Pro V3 Internal SSD Mod for the method I used to add a completely internal SSD to the Pogoplug Pro V3. Notice I have also done the external antenna mod for the wireless NIC onboard so both can coexist at the same time. The hardest part to the whole mod was soldering the 5 volt wires to the front USB port but once that is done the whole assembly fits in snuggly (almost perfectly) without need for fastening or case modification. Note you can get a msata drive in a capacity up to 1TB which would fit in the adapter I have used.
Here is a link to the cable I bought that made this work: 22 Pin Sata Cable with 5 volt only supply
Also, a link to the adapter I used: MSATA to 22 Pin SATA Adapter
And here is an eBay search for 24GB drives which will fit in the MSATA adapter: 24GB SSD's on eBay US
And if you wanna "go all the way', here is a 1 Terabyte drive you could use: Samsung 1TB MSATA Drive
Here is a link to the cable I bought that made this work: 22 Pin Sata Cable with 5 volt only supply
Also, a link to the adapter I used: MSATA to 22 Pin SATA Adapter
And here is an eBay search for 24GB drives which will fit in the MSATA adapter: 24GB SSD's on eBay US
And if you wanna "go all the way', here is a 1 Terabyte drive you could use: Samsung 1TB MSATA Drive