there is one method to get oldstyle figures with my module (which i showed him)
and should use, if he don't like this method the alternative is to define his own
fontfeatures and use them (\set...font[...][features=myfeature])

I do use the method that you suggested. I only reported the problems with the other methods because I didn't know whether the behaviour was expected or caused by some bug that you or Hans might want to know about.