Another Browning specialist informed me that, as far as he knew, the Baylor letters project is “free to air”: excellent news. He also recommended the book The Courtship of Robert Browning and Elizabeth Barrett by Daniel Karlin (1985, Oxford: Clarendon Press) . It draws on the letters, but it isn’t a linguistic study – which would therefore seem a welcome and potentially very interesting addition to current Browning scholarship.
Daniel Karlin also publised an edition of selected courtship letters (1989, Oxford: Clarendon Press). Both books are available in the Leiden University Library.
You will find more information on the Browning letters project here. You can also subscribe to a regular update of the collection.
Thanks for your information, John!