Certainly runs on Raspberry Pi - there are several mentions to that effect.

Sadly I can't help you more - best ask on mailing lists[0] or on IRC[1].

Historically Plan 9 was developed from grounds up to support several architectures, so in this case you're really asking about drivers for specific peripherals.

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[0] http://mail.9fans.net/listinfo/9fans and http://lists.9front.org/

[1] #plan9 and #9front and #cat-v on irc.freenode.org