Topic and comment in T4

A question thrown into the air:

In replying to And's post in the 're: telona' thread, I've noticed
that the design of my T4 (= Telona) draws an equivalence between
head-first phrase structure and topic-before-comment sentence
structure, by defining the meaning of a sentence to be the assertion
of the existence of a referent for the sentence-phrase.  I wonder
whether this is watertight, or whether there might be times when the
head of a phrase might not become the topic of the corresponding
existential assertion?

Jonathan.
--
'O dear white children casual as birds,
Playing among the ruined languages...'
Auden/Britten, 'Hymn to St. Cecilia'