Great you are supporting her on this. My daughter is similar and I too want to aim for grammar. RHS seems suitable, it is good all round not just academic.
In terms of meeting with the head I would just let her know your intentions, if she has any experience of a dyslexic child taking the 11+ from her school and if she will support your application for extra time.
There isn't much they can do to help other than that. For you, get your tutor booked. I used Gloucestershire Dyslexia Association for general tuition for a year (my DD diagnosed privately beginning of Y3). We now just have a general tutor and are starting to do a bit of VR.
Do have a back up school though. But have a go, why not ? if you would like to know the tutor I use PM me. I am not sure which part of the Shire you are from ?
Good luck with your meeting. State your intentions and see what they say.