Try this, open up the save game in notepad (it will be the .eug file)
you'll find something like this which is I think the default
ENG = { desc = "" countrytactics = "" playername = "United Kingdom : " }
FRA = { desc = "" countrytactics = "" playername = "France : " }
GER = { desc = "" countrytactics = "" playername = "Germany : MORT" }
RUS = { desc = "" countrytactics = "" playername = "Russia : " }
TUR = { desc = "" countrytactics = "" playername = "Ottoman Empire : " }
HOL = { desc = "" countrytactics = "" playername = "Netherlands : " }
SWE = { desc = "" countrytactics = "" playername = "Sweden : " }
SPA = { desc = "" countrytactics = "" playername = "Spain : " }
USA = { desc = "" countrytactics = "" playername = "USA : " }
CHI = { desc = "" countrytactics = "" playername = "China : " }
(Computername MORT, playing as Germany)
Replace it with something along the lines of
AWA = { desc = "" countrytactics = "" playername = "Awadh : " }
BHU = { desc = "" countrytactics = "" playername = "Bhutan : " }
NIP = { desc = "" countrytactics = "" playername = "Japan : SECTOID" }
Computer name here is SECTOID, they were the only handy saves I have right now sorry. I don't think the computer name matters, just use whatever is your computer name.
Now the greyed out countries will be Awadh, Bhutan and Japan will be selectable. You could even just go with replacing the whole block with
NIP = { desc = "" countrytactics = "" playername = "Japan : SECTOID" }
Which makes any country selectable, BUT on the stats screens the only nations that show up are the ones listed here.
I *think* that is all you need to do, but haven't time to test.