Greetings Marion,
Planting memorial trees in someone's honor has become a more common tradition and practice over the last decade...personally, I think it is a wonderful idea and a symbolic gesture!
Yes, planting trees in late October and early November is reasonable for central Illinois. We usually only plant ball and burlap trees in autumn. With that said, I would encourage you to contact the Urbana Park District or even the Champaign County Forest Preserve District to determine whether they would honor your request to plant a memorial tree on their property.
Urbana Park District: (217) 367-1544
Champaign County Forest Preserve District: (217) 586-4389
Hope this helps you get started with this wonderful idea!