@remcolmaybe you are wanting to use _paq.push(['setCustomTitle', '']) ? That might help. Otherwise you basically need to set up a variable in tag manager that reads that data layer value and assign this variable to the Custom Title field in the Matomo Configuration Variable. Depending how your knowledge is re tag manager the paq.push method may be easier.
I do get your suggestion about setting up a variable in the tag manager that reads that data layer value and assign this to the Custom Title field in the Matomo. I will give that a try.
@remcol I was suggesting _paq as a workaround / possible solution as the Matomo Tag Manager includes the Matomo Tracking code and you could basically bypass the tag manager using _paq and change the custom title directly this way. I haven’t tested it, but I would assume this works.