I use DreamWidth as a mirror-and-posting site for LJ, because it's trivially easy to auto-crosspost from there to here but not the reverse. I import comments from here to DW every few months, so that the mirroring stays fairly current.

If you look at one of my posts here, you'll see some footer text giving a link to the corresponding DW entry. You can set that on DW by going to Organize --> Manage Account --> Other Sites. If you want to force people to comment on the DW side, you can also set a switch there to disable commenting on cross-posted entries.