After downloading the 30-day trial version, I think I can answer my own question.
Sadly, Aperture seems to completely ignore any XMP metadata. Thus any metadata stored in XMP in a DNG file will not show up in Aperture. Likewise, any metadata added to an image in Aperture will not be written in XMP format in an exported DNG file; the metadata will only show up in legacy IPTC format.
This is odd as Apple normally prides itself on being progressive and adopting industry standards. ITPC has been a supporter of XMP for some time and recommends it in place of the legacy format.
At least Apple is consistent. OS X also completely ignores XMP data in sidecars and in DNG files. Spotlight does not see XMP data.
Lightroom does support XMP and seems to work well with DNG files.