@Tailbetweenlegs
"I walked him in his new outfit today and a man rolled his eyes and dramatically jumped out of the way"
Personally I think that makes sense, especially if the dog was unmuzzled. I do this when I see dogs, partly because I want to be as far away as them as possible, and partly because most dog owners fail to realise that many people detest dogs, and I want to try to get it through to them that some people are really uncomfortable.
"a woman stopped me to ask if he was nervous (really!)"
She sounds very stupid!
"a woman with a dog walked right past with her dog staring at mine"
The dog was staring or she was staring? I tend to stare at dogs to maximize my reaction time in case they decide to attack me. If a dog is anywhere near me I want it in my direct eyeline (if it is securely tied up to something solid and muzzled I am less wary, but I don't even assume that the owner is capable of holding onto the lead, let alone anything else).
"So should I not bother with an I NEED SPACE lead/harness. I’m interested to hear how it comes across and what you’d think if you
saw a dog in this/if you’d notice."
I'm not sure. It is partially very considerate of you, but it does lead me to wonder why the dog isn't muzzled and is out in public.
"I just mean it as a warning so people give us a bit of a wide berth so they aren’t startled if they get barked at by quite a big dog."
Why aren't you giving people a wide berth if your dog is the problem? Barking dogs startled and scare whether or not you're giving them a wide berth. Why is your dog in public if it scares people?
"I don’t expect people to go massively out of their way to avoid us"
How bloody generous of you. I was wondering whether you might like to phone ahead and ask the park rangers to clear the park of all other dogs and humans before you get there! Seriously - your dog is the problem, keep him as far away from the public as possible please.
"I’m worried it comes across as passive aggressive or will invite attention which is the opposite of what I want"
There is a risk of it attracting dog-lovers who want to know more, and it is also pretty cheeky - you're giving a warning and telling people to stay away, when really you shoul dbe the one taking 100% responsibility for your dog not scaring or attacking others.