Cannot reach the bearing adjustment nut on rear wheel
You could adjust the other end instead, so long as the cassette locknut is already tight against the cone (if it isn't, take the cassette off and do that first).

http://sheldonbrown.com/cone-adjustment.html:
"For cassette hubs, or conventional rear hubs that I want to adjust without removing the freewheel, I use a thin 15 mm wrench and the two 17's.

To loosen this type of hub, I usually do have to loosen the left locknut from its cone, loosen the cone, then re-tighten the locknut. To tighten it, the two 17's on the locknuts do the job quickly and easily."