Use the adjust-copy-to preprocessing extension with the base XHTML transform
This plugin does nothing except call the XHTML transform but with the dc-preprocess Ant target used in place of the base preprocess Ant target. This has the effect of applying the adjust-copy-to (and any other preprocessing extensions) to the XHTML.
Requires the org.dita-community.preprocess-extensions, org.dita-community.adjust-copy-to, and org.dita-community.common.xslt plugins.
See the org.dita-community.adjust-copy-to project's readme.md file for details.
NOTE: This plugin will only work with the 1.8.5 Open Toolkit.