Loops to create new variables in ddply

I am using ddply to aggregate and summarize data frame variables, and I am interested in looping through my data frame's list to create the new variables.

new.data <- ddply(old.data, 
                  c("factor", "factor2"),
                  function(df)
                    c(a11_a10 = CustomFunction(df$a11_a10),
                      a12_a11 = CustomFunction(df$a12_a11),
                      a13_a12 = CustomFunction(df$a13_a12),
                      ...
                      ...
                      ...))

Is there a way for me to insert a loop in ddply so that I can avoid writing each new summary variable out, e.g.

for (i in 11:n) {
  paste("a", i, "_a", i - 1) = CustomFunction(..... )
}

I know that this is not how it would actually be done, but I just wanted to show how I'd conceptualize it. Is there a way to do this in the function I call in ddply, or via a list?

UPDATE: Because I'm a new user, I can't post an answer to my own question:

My answer involves ideas from Nick's answer and Ista's comment:

func <- function(old.data, min, max, gap) {
  varrange <- min:max
  usenames <- paste("a", varrange, "_a", varrange - gap, sep="")
  new.data <- ddply(old.data,
                    .(factor, factor2),
                    colwise(CustomFunction, c(usenames)))
}