The term "custom schema support" frequently is used to describe the possibility to interleave an office application schema with XML tags from some other schema. Because this is a feature of XML and XML Namespaces in general, it is supported by OpenDocument. It is important to distinguish between the OpenDocument format and applications that implement it, however. No applications at this point exploit this feature, but this is inherently supported by the OpenDocument format.
Another definition of "custom schema" support is the possibility to include an instance of a non-office-schema into an office document. This feature is provided by OpenDocument, due to its partial inclusion of the W3C XForms Recommendation.