I agree with Dan.
We print field guides on a poly paper with no other coating and they stand up to some of the worlds harshest conditions, every where from alaska to the middle east as well as withstanding gas, oil, and other various chemical exposures. i think toner will be fine, paper type will be your key to success.
Just as a reference we use a 5 mi and 10 mil poly stock.
As the toner is plastic it should hold longer than the paper would if exposed to the elements. My suggestion would be to UV coat or Laminate it. There are Matte UV fluids. If you use polyester paper you may not need to worry for then your printing plastic on plastic and that will last.
I have a project that will require a peice printed on a igen4 using matte toner to withstand the elements for up to a year with exposure to saltwater and solvents. Any studies of toner holding up to the great outdoors?