I honestly don't think there is a chemical that would deter ticks significantly. The last carcass I took home I did a bit of an experiment with and hung it over a tray of water over night to catch any stowaways jumping ship and was amazed at the payload that had tried to jettison and been trapped in the water the next morning!
I had checked around the genitals and usual places for ticks and whilst they were there as expected it didn't look excessive. Let me tell you, the water was literally covered in ticks (from adults to tiny nymphs), keds and mites.
I also put the head (looked fairly clean) into a clear plastic bag and put a squirt of insecticide in and sealed it. The bag was riddled with more tick nymphs, mites and keds the next day.
Makes you think about how many bail out, when you do the gralloch, in the boot of the car etc!....
There were a few ticks still attached 2 days later whilst skinning that for some reason hadn't realised the host had expired. I could see the inflammation on the inside of the deer's skin where the bite was.... some of these ticks had died in situ too for some reason?! The real eye opener was finding the odd tick still on the carcass that had crawled to a high point to wait for the next host. Just passing my hand in front of them was enough to stimulate them to reach out to grab hold. I'm convinced they first look for a heat source and then once aboard look for somewhere palatable to bite (dark, not noxious and next to skin). Which makes me think that chemicals are not going to deter them from hopping aboard, merely make them look for some exposed skin, which they are going to do anyway.
I reckon your best defence is mechanical. If your trousers are tucked into gaiters, you are wearing wellies etc then you are going to make it hard for them to find somewhere to get stuck in.
The second line of defence is vigilance. These arachnids are excellent at what they do. Their whole existence depends on food 'just walking on by' they are not going to pass up an opportunity if they can help it. We are all going to get bitten at some point and most likely a lot of us already have without ever having known. Be vigilant, but don't panic if you do find one. Just remove it in the prescribed way and keep an eye on it. Also keep an eye on what looks like mosquito bites as they could have been left by a tick that has already jumped ship.
They deserve respect certainly, but common sense and a 'weather eye' should keep them at bay!
Alex