The current version of OME.xsd (2016-06) will not change in any breaking way. Full stop. If such a change is needed, a new major version of the XSD, e.g. 2019-10, would be released and there would also be an XSL transform provided for upgrading from 2016-06 to 2019-10, cf. 2015-01-to-2016-06.xsl, as well a transform to downgrade, cf. 2016-06-to-2015-01.xsl.
This means that the strategy you outline should produce OME-XML which OME intends to always be interpretable. That being said, there is a good deal of metadata that isn’t going to have a natural home in OME-XML, which is a large part of the discussion in Data archiving: what format to use, which files to keep?
This problem is one of the drivers for the work discussed in Metadata in RDF and Proposal for 4D Nucleome Microscopy Metadata Guidelines based on the OME data model. In other words, yes, the OME Model is likely to evolve in the near future in order to allow more reliably capturing vendor-specific metadata as well as allowing community members to define their models. And I very much assume that your use case would be a great match.
All the best,