I would be among those suggesting a hat cover, if you want to go for a more traditional look in Pleasure Driving. I have seen some that are outstanding in appearance, would be easily mistaken for a hat, not helmet.
I am among those that say "no glue on a helmet", which I learned could affect the integrity of the plastic, while working in Industrial settings. Hardhats COME with warnings not to write on them, add stickers, etc., or they could not protect your head with failure of plastics from chemical interaction.
Perhaps you could get a helmet cover, decorate it and then use it over your helmet at the show. I checked with DD Rapps a couple years ago, and they did glue on stuff to the helmet, so not an idea I could use. Might have some new ideas, methods, they do now.
Getting some hats at Goodwill or Thrift stores could let you experiment inexpensively, by removing or steaming and STREEETCHHING out the felt crown, brim, so it fits over the helmet when sewed on a hat cover. Similar to the Driving Hideahelmet photos in the link Drive NJ posted. Those look nice. Maybe they just slip onto the helmet, not glued on so you can change your look often. You also might find some good flowers or feather decorations on those Goodwill hats, transfer them to the helmet cover.
You can also get some nice clothes to show Pleasure in at those stores, you don't worry if horse slobbers on a $5 jacket!
With our pony sometimes we braided, sometimes not. He has a very frizzy mane that cannot be forced to lie flat on his neck. Some judges liked it, one told us we would place higher it, instead of going head to head against those great little welsh ponies, we should skip the braids and play up what a cutie Dublin can be.
I think having enough interest in having a stylish, comfortable, and practical helmet cover can lead you to design your own with different innovative ideas. One of the way can be helmet covers with brims on them.