Respondendo minha própria consulta:
Não é uma resposta definitiva para a pergunta, como eu pedi, mas eu achei este artigo , cobrindo o tema das ilustrações de Gorey para as histórias de Bellairs.
As citações principais:
However, despite their interwoven legacy in the minds of readers, Bellairs and Gorey never met. A member of The Edward Gorey House Museum in Yarmouth, Mass., went so far as to say that they may never have corresponded.
Andreas Brown who helped Edward Gorey establish the Edward Gorey Charitable Trust (and who was a friend of Gorey for many years) acknowledged this to likely be the case. “I haven’t seen any correspondence between them in the archives. They would send him the manuscript and he would read it and then do a cover that he thought was appropriate for the story,” Brown told me in a phone conversation. “It doesn’t mean there couldn’t be two or three letters in miscellaneous correspondence. But as far as we are aware, they never communicated.”
...
In fact, according to Brown, later in his life, Gorey wanted to disown his cover illustrations for Bellairs. “He called me up one day and said, ‘Let’s get all of the Bellairs work out of the archives.’ He just didn’t think it represented him and what he was trying to do. He saw it as his grunt work.”
Isso certamente demonstra que não houve colaboração ativa entre os dois criadores, embora não indique exatamente como Bellairs abordou sua escrita, em relação ao trabalho de ilustração de Gorey.