I found a better approach, one that isn’t as scary …
builder.Define(
mEntryColumn,
aEditor,
(column, editor) => column.AddNumericField(editor.Format)
.BoundTo(editor, e => e.Value)
.FocusBoundTo(editor, e => e.Focused));
The key difference is one of approach - instead of trying to subscribe to an arbitrary event (which is difficult since you can’t pass events around as data, only delegates), I went with subscribing to an arbitrary property and keeping the gnarly bits under the covers.