No, there's nothing to stop you talking to the grammar school. The only thing forbidden - for fairly obvious reasons - is that the school being appealed for cannot provide support for any appellant at a hearing:
Panels must not allow representatives of schools to support individual appeals for places at their school at the hearing itself or by providing letters of support for appellants, because of possible conflicts of interest and the possibility of unfairness to other appellants.
It's up to the school, of course, whether they wish to talk to parents about a prospective appeal!
The only other point I would make is that appeals are more often than not dealt with by just one person at a very senior level within the school (e.g. the headteacher or one of the deputy/assistant heads). It's possible that the head of year may not know much about the subject.