All ampersands that are returned from getLabel() in an OptionModel get encoded to &. This makes it impossible to use any html entities (— etc) within option labels in selects. This is not that uncommon and should definitely be supported by Tapestry out of the box.
It can be fixed by changing line 60 of org.apache.tapestry5.internal.util.SelectModelRender from
This would mean that ampersands will need to be encoded for option labels, but this is at least possible for a developer to do. Currently it's not possible to use entities without using a custom select component, you can't even extend the T5 Select class and override the Render nested class.