Mckenzie area is one of my favorites.
During that time of season bring bug protection in the form of head nets and Deet or comparable coverage. You should be almost completely clear on the McKenzie River National Trail. If you plan on going up Old McKenzie pass you may run into a closed road about 7 miles up as it usually doesn't open up until early July. During that time of year I set up an excel sheet to track snow melt and check the snotel maps http://www.or.nrcs.usda.gov/snow/map...n_sitemap.html
to determine snow melt rate and plan around that. Its a great little area you should enjoy it.
Also you shouldn't rule out Hwy 20, Hwy 22 or Santiam Pass there are many great trails within about 45 minutes of where you are planning on going.
Some highlights to hit.
Sahalie and Koosah Falls
Deer creek hot springs (Clothing Optional) if the water level is good enough.
Bagby Hotsprings (Clothing/Fee required)
That is my old stomping ground so if you want any more in depth info give me a holler.