OK, I have gotten a bit further, so I have another question:
To me, it looks like the best approach would be to have another option in the markup settings.
( /@@markup-controlpanel )
Since my 'tags' probably 'technically is plain text', I wonder:
Can I invent a mime type and add it ?
PS: I did try:
<registry>
<record name="plone.allowed_types" interface="Products.CMFPlone.interfaces.controlpanel.IMarkupSchema" field="allowed_types">
<field type="plone.registry.field.Tuple">
<default>
<element>text/x-xtags</element>
</default>
<value_type type="plone.registry.field.Choice">
<vocabulary>plone.app.vocabularies.AllowableContentTypes</vocabulary>
</value_type>
</field>
Module ZPublisher.Publish, line 138, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module Products.CMFPlone.controlpanel.browser.quickinstaller, line 687, in __call__
Module Products.CMFPlone.controlpanel.browser.quickinstaller, line 452, in install_product
Module Products.GenericSetup.tool, line 388, in runAllImportStepsFromProfile
__traceback_info__: profile-collective.transform.xtags:default
Module Products.GenericSetup.tool, line 1433, in _runImportStepsFromContext
Module Products.GenericSetup.tool, line 1245, in _doRunImportStep
__traceback_info__: plone.app.registry
Module plone.app.registry.exportimport.handler, line 79, in importRegistry
__traceback_info__: registry.xml
Module plone.app.registry.exportimport.handler, line 125, in importDocument
Module plone.app.registry.exportimport.handler, line 245, in importRecord
__traceback_info__: record name: plone.allowed_types
Module plone.supermodel.exportimport, line 148, in read
Module zope.schema._bootstrapfields, line 52, in __set__
Module zope.schema._bootstrapfields, line 183, in validate
Module zope.schema._field, line 527, in _validate
WrongContainedType: ([ConstraintNotSatisfied('text/x-x-tags', '')], '')