Yes it should be fine, and is a sensible choice. I always wore a beagler on Dutch but I can understand and I would recommend someone wears a proper hat, especially with a young horse.
My hat is a similar style (Champion Ventair) and I've wondered about this. I was thinking if I decide to do affiliated dressage in it, I'd look like an eegit who'd wandered in by accident! Am I right or do you see these in affiliated competiton at the lower levels too?
Only thing you will have to worry about is if you do any comp organised by a PC or a RC is that they will insist on you wearing a kitemarked hat, which it doesn't say it is.