Reserved prefixes can be declared in the package document, but currently I get this error:

ERROR(OPF-007b): .\content\30\epub30-test-0300.epub/EPUB/fxl-package.opf(2,143): Invalid prefix mapping: default vocabulary 'rendition' must not be re-declared.

There should only be a warning if you define a reserved prefix, as publications may implement prefixes before they get standardised:

Reserved prefixes should not be overridden in the prefix attribute, but Reading Systems must use such local overrides when encountered.