data.table efficient alternative to grouped assignment as DT[ ,x:=f(y),by=z]?

I am looking for the best alternative to the not yet implemented (to my knowledge) assignment by reference in a data.table by groups. Using the data.table example,

DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
     x y v
[1,] a 1 1
[2,] a 3 2
[3,] a 6 3
[4,] b 1 4
[5,] b 3 5
[6,] b 6 6
[7,] c 1 7
[8,] c 3 8
[9,] c 6 9

I want to add a new column z, containing f(y,v) grouped by values of x (lets take f(y,v)=mean(y)+v). Note that I do not want to print or store the result of this computation as in

DT[,mean(y)+v,by=x]
      x        V1
 [1,] a  4.333333
 [2,] a  5.333333
 [3,] a  6.333333
 [4,] b  7.333333
 [5,] b  8.333333
 [6,] b  9.333333
 [7,] c 10.333333
 [8,] c 11.333333
 [9,] c 12.333333

but rather I want to add the result to DT:

     x y v        V1
[1,] a 1 1  4.333333
[2,] a 3 2  5.333333
[3,] a 6 3  6.333333
[4,] b 1 4  7.333333
[5,] b 3 5  8.333333
[6,] b 6 6  9.333333
[7,] c 1 7 10.333333
[8,] c 3 8 11.333333
[9,] c 6 9 12.333333

my data.table has 262 MB, such that

DT <- DT[,transform(.SD,mean(y)+v),by=x]

is not an option, since I cannot fit DT twice in memory (which is implied by the copy operation, I think). Fact is I've never seen that operation finish.

What alternatives do I have (until data.table comes with DT[,z:=mean(y)+v,by=x])?

I just read about DT[newDT]. What's wrong here?

newDT <- DT[,mean(y)+v,by=x]
      x        V1
 [1,] a  4.333333
 [2,] a  5.333333
 [3,] a  6.333333
 [4,] b  7.333333
 [5,] b  8.333333
 [6,] b  9.333333
 [7,] c 10.333333
 [8,] c 11.333333
 [9,] c 12.333333

(which is doable memory wise.) then:

> DT[newDT]
setkey(DT,x)
setkey(newDT,x)
x y v        V1
a 1 1  4.333333
a 3 2  4.333333
a 6 3  4.333333
a 1 1  5.333333
a 3 2  5.333333
a 6 3  5.333333
a 1 1  6.333333
a 3 2  6.333333
a 6 3  6.333333
b 1