"NAMEPLATE_FONT" is just a global variable that points to a font file; it's defined somewhere in Blizzard code as "Fonts\\SomeFont.ttf". Your modifications look correct, and you said they do affect the font, so what's the problem?
I'll have to apply the changes and give it another shot- I gave up for a bit so my description is from memory.
At first, everything worked; the nameplate was textured, recolored and even the font was set correctly. If I turned the camera away from nameplates (to look at the sky for example) and then returned it, the nameplates would no longer have the texture, font or even coloration. Caellian responded a few hours ago to another person who was asking how to go about changing the font; he explained the exact method I used.
I'll give it another shot - hopefully I won't have the oddity occur.
EDIT: Yes, this still occurs as I've described. Very odd. :\ I also did a UI reload and managed to capture some errors, though they baffle me:
[20:45:28] Interface\AddOns\caelNamePlates\caelNamePlates.lua:73: <unnamed>:SetText(): Font not set
[C]: in function `SetText'
Interface\AddOns\caelNamePlates\caelNamePlates.lua:73: in function <Interface\AddOns\caelNamePlates\caelNamePlates.lua:38>
Interface\AddOns\caelNamePlates\caelNamePlates.lua:164: in function <Interface\AddOns\caelNamePlates\caelNamePlates.lua:89>
Interface\AddOns\caelNamePlates\caelNamePlates.lua:188: in function <Interface\AddOns\caelNamePlates\caelNamePlates.lua:178>