I believe you have to get permission from the Park Service to solo.
From: http://www.nps.gov/mora/planyourvisit/climbing.htm
"Solo travel above high camps or anywhere on glaciers is not permitted except with prior written permission from the Superintendent. You may download a Solo Climb Request Form http://www.nps.gov/mora/planyourvisit/upload/solo%20climb%20request.doc (Word document, 83 KB) or you may request this form by writing: Superintendent, Mount Rainier National Park, 55210 238th Avenue East, Ashford, WA 98304.
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