You can set the html readonly attribute using custom attributes. This is a new feature in Domino 8.5.3.
In all properties for the field, there should be something named attrs. Add a new attribute, name readonly and value readonly.
E.g.
<xp:inputText>
<xp:this.attrs>
<xp:attr name="readonly" value="readonly"></xp:attr>
</xp:this.attrs>
</xp:inputText>