I agree - it would actually be quite improper for the panel to seek information from the school, and for the school to release such information without your agreement.
It seems to me, though, that a missing year 5 report only becomes an issue if it's the last available full report. Take the independent sector, for example, where full reports are likely to be issued two or three times a year. An appeal panel, presented with a full (year 6) Spring Term report, is not going to query where the year 5 report is.
Provided that your school will provide a very full and detailed report at the end of the spring term - or early in the summer - that should suffice.
Some year 5 reports I've seen are close to useless for appeal purposes (hardly anything about achievement, but large chunks explaining what has been covered in the syllabus!). I make some suggestions here about the sort of information an appeal panel might like to see:http://www.elevenplusexams.co.uk/appeal ... cation#b48
Not saying that all of this would fit your circumstances. If this is intended as a substitute report (as well as serving as a letter of support), there ought to be up-to-date comments on all the individual subjects. It could also be helpful if the headteacher were willing to use the line "if the test had been 6 months later she would have passed with flying colours