Hi Elaine -
Interesting questions. Most of what I'm writing here is inference and I've not found a definitive guide (yet) online. First, the poison ivy plant itself is not the issue but a secondary compound found within resin canals in the plant is the problem. Urushiol oil is the chemical which will bond with our skin and can cause an immune system response. The oil is not on the leaves but in the leaves, fruit skin, stem, vine, and roots. If the leaf is still intact, then it will not "pass on" any urushiol oil. Unfortunately for us, the leaves are quite fragile and easily broken. The urushiol oil then can escape the plant - often you'll see this as black dots on the leaflets. Urushiol oil will oxidize fairly quickly turning yellow, brown, then black.
If the leaflets touched the mushrooms but were still intact (not broken or damaged), then there should be no passing of urushiol oil to the mushroom. If the poison ivy leaflets are damaged, then yes there could be a passing of urushiol oil onto the mushroom. Consuming urushiol oil can cause a serious reaction and may even be fatal.
Urushiol is a heavy oil. It can be made airborne under certain conditions. It is important to note that urushiol oil is not volatile under heated conditions. In the case of burning poison ivy plants, droplets on dust particles and ash are the means of transmission not from vapors. So as I understand it, no - the heated drying process would not cause the urushiol to become airborne.
As I mentioned before, urushiol oil does oxidize and is distinctive. If you see black dots that you don't know the origin of and suspect urushiol oil - it could be. Urushiol oil is pretty easy to wash off but if I remember correctly, mushrooms are fairly fragile and you generally don't want to wash. This is something you'll have to decide.
Your best defense here is to know how to identify poison ivy plants positively. That way you can keep yourself safe from accidental contact. FYI - a case in point about touching undamaged leaflets. I was picking blackberries in shorts this past Friday and looked down at one point to see that I was brushing up against poison ivy plants. These were young, healthy plants with no visible damage. I was too far away from a water source and so just figured if it was going to happen it would. It didn't. The casual brushing was not enough to damage the leaflets and so no rash. There has to be damage to the plant in order for the urushiol oil to be released.
I hope this answers some of your questions.
Here are some references I came across while looking for more information to answer your question:
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