--->UPDATE<--- Aperture Exporter is now compatible with MacOS 10.14 Mojave.  You can get the newest version (1.4.0) here right now and is available for purchase directly from us.  We are unsure of when we will return to the Mac App Store.


Embedding XMP Sidecar metadata into JPEGs

XMP Sidecar files can optionally be generated for JPEGs via an advanced option.  While there are many advantages to storing metadata in these sidecar files there is one big drawback.  Adobe's Lightroom does not read them for JPEGs.
This is typically not an issue as in its default configuration Aperture Exporter embeds metadata directly into JPEGs.  This approach works well in most situations for most people using most cameras.  However, when issues do arise it becomes most apparant when examine the "originals" that were exported via AE and the "originals" that are either referenced or managed by Aperture.  For details about what can go wrong see our support page entitled JPEG Original Exported By AE Differ From The True Original.  Note that this issue is not limited to AE.  It is also a problem when using Aperture's built in feature that Writes IPTC Metadata To Original...
The following describes a method of ensuring that the maximum amount of metadata is retained from the true "original" while merging in other metadata added or modified in Aperture is also retained.  In summary you will configure Aperture Exporter to generate XMP Sidecars for JPEGs in advanced options.  Now go ahead and export your images using Aperture Exporter.
In the next step we will take the XMP Sidecars and embed their metadata back into the JPEGs, thus making it compatible with Lightroom.  This will require the use of a command line tool called exiftool.  

ExifTool is a platform-independent Perl library plus a command-line application for reading, writing and editing meta information in a wide variety of files. ExifTool supports many different metadata formats including EXIFGPSIPTCXMPJFIFGeoTIFFICC ProfilePhotoshop IRBFlashPixAFCP and ID3, as well as the maker notes of many digital cameras by CanonCasioFLIRFujiFilmGEHP,JVC/VictorKodakLeafMinolta/Konica-MinoltaNikonNintendoOlympus/EpsonPanasonic/LeicaPentax/AsahiPhase OneReconyxRicohSamsungSanyoSigma/Foveon and Sony.

ExifTool is also available as a stand-alone Windows executable and a Macintosh OS X package:  (Note that these versions contain the executable only, and do not include the HTML documentation or other files of the full distribution above.)

It is freely available from http://owl.phy.queensu.ca/~phil/exiftool/

I’ve done some work with exiftool looking for a easy workflow to injest the XMP sidecars into the JPEGs.  It is very feasible.  To do so I do the following:
  1. Backup the folder to which you exported your Aperture library.  You might consider this to be optional because if something goes wrong you could always perform another export using AE.
  2. Open terminal.app
  3. Use the following command in terminal.app
exiftool -overwrite_original -tagsfromfile %d%f.XMP -all:all -ext jpg -ext JPG -r DIR
Where DIR is the root directory you want exiftool to start working on.  For example:  /path/to/your/AE_EXPORT_DIR
This command line tells exiftool to process all the JPEG images in the exported folder and subfolders.   It will read the matadata in a XMP sidecar, then write it metadata structure of the JPEG.  Note that this does not affect the image itself, so you don't need to worry about loss due to recompression. In my experiments this provided for excellent results, maintaining the existing metadata and adding/modifying the metadata as reflected in the XMP sidecar.
Let me know if you try this tip out and if it works out well for you.