Cartesian rolling join with data.table - r

Question

I'm not exactly sure how to describe this, but I'll gladly edit the title and/or post to reflect comments and answers.
Problem
I have two data.frames that I would like to merge with a combination of a left join, an outer join, and a rolling join.
One of the key columns (year) is for the rolling join.
Another key column (cat) is common to both data.frames. In the example below I've only supplied exemplary subsets of the full data, which has thousands of values for cat.
The first data.frame, X, has an additional key column cnty (county), and the second data.frame, Y, has an additional key column pol (pollutant).
For each group defined by cat and year, I would like the final result to contain a cartesian product of cnty and pol, with value columns emfac (from X) and tput (from Y). The goal is to be able to compute emfac * tput.
Here is an exemplary subset of X:
cat year cnty tput
1 29 2011 ALA 67852
2 29 2011 CC 33893
3 29 2011 MRN 11319
... and here is an exemplary subset of Y:
cat year pol emfac
1 29 1975 TOG 2.4
2 29 1975 PM 5.3
Closest attempt so far
I can almost, but not quite, get the output I want:
X <- structure(list(
cat = c(29L, 29L, 29L),
year = c(2011L, 2011L, 2011L),
cnty = c("ALA", "CC", "MRN"),
tput = c(67852, 33893, 11319)),
.Names = c("cat", "year", "cnty", "tput"),
class = c("data.frame"), row.names = c(NA, -3L))
Y <- structure(list(
cat = c(29L, 29L),
year = c(1975, 1975),
pol = c("PM", "TOG"),
emfac = c(2.4, 5.3)),
.Names = c("cat", "year", "pol", "emfac"),
class = c("data.frame"), row.names = c(NA, -2L))
library(data.table)
X <- data.table(X, key = c("cat", "cnty", "year"))
Y <- data.table(Y, key = c("cat", "pol", "year"))
Y[X, roll = TRUE]
cat year pol emfac cnty tput
1: 29 2011 PM 5.3 ALA 67852
2: 29 2011 PM 5.3 CC 33893
3: 29 2011 PM 5.3 MRN 11319
This is my "nearest miss". Most of my other attempts are much more wrong.
Expected result
cat year pol emfac cnty tput
1: 29 2011 PM 5.3 ALA 67852
2: 29 2011 PM 5.3 CC 33893
3: 29 2011 PM 5.3 MRN 11319
4: 29 2011 TOG 2.4 ALA 67852
5: 29 2011 TOG 2.4 CC 33893
6: 29 2011 TOG 2.4 MRN 11319
What am I doing wrong?