I've actually done this a number of times, but with EDLs (also AAFs from MC). The preferred route is QuickTime media and a corresponding EDL. You could go with EDL-X, but the shop I work out of a lot doesn't own it. Also, it's good to aggregate the media used instead of sending them all files or trimmed files. So this is the process I followed. It sounds a bit convoluted, but it isn't.
FCP X -> export FCPXML
Import that into Xto7 and send to FCP 7
From FCP 7 generate new XML
Load that into Automatic Duck Media Copy (free) - this copies the used files in their entirety
Open the new XML as an FCP project and relink to the copied files
Generate EDL from FCP 7 and send with the copied QTs to the Baselight colorist
(If you have EDL-X, you can skip some of these steps, of course.)
Baselight will render unique media with handles. These files are renamed - usually with a suffix - in order to separate segments rendered from a single longer clip. I think they can send a new XML to correspond to the new renders, but you should check. If not, then relinking may require a bit of manual effort. When I had to do this, I used Better Renamer to strip off the suffix, with a handful of exception where clips came from the same original file. This meant relinking in FCP 7 was automatic, with only a few exceptions that had to be linked manually.
Double-check that they can work with the same codecs. I believe ProRes is an extra paid option with Baselight.
Once you have relinked the new rendered files in FCP 7, export an XML.
Open in 7toX and send to FCP X.
You could. I prefer to do this outside of FCP X to keep the files unaltered. In any case, you still have to end up with an FCP 7 XML or EDL. So dupe+organize and EDL-X would work too, but I haven't tested it.