According to the link below, Issac Bruce wants to wear Jerry Rice's number with San Fransisco. At the very bottom of the article there is a link to Rice giving Bruce permission to wear the jersey.
Turnip is right that his jersey was never retired although it definitely should have been...even if it was Bruce could still wear it with permission. Rice wore #80 in Seattle thanks to Largent's generosity and Bruce was respectful enough to talk to Rice before taking the number in SF.