I’ve contacted Flåm and Hugastøl tourist centre (thanks for the advice, Matthew). Both said Rallarvegen is rideable, but they weren’t particularly recommending it because of the risk of falling rocks.

I’m gonna ride up to Finse (from Flåm) tomorrow and then continue to Haugastøl and Hardangervidda the day after. I’ll post an update with my own observations then. ?