And I'm still not thinking clearly.

The risk of having Patil write to Seamus is that it tips our hand rather clearly to the existence of a spy in that meeting, and even if we could trust Seamus to keep his mouth shut, which we probably can't, he's regularly in the presence of Bellatrix, who's a legilimens.

Probably best not to. Well, we could encourage Haruman to let her write to him and ask how things are going.