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I've done this with a 9spd fsa chainset friction shifting over a 7spd block with no issues
Although you're not guaranteed to have 'no issues' when using 9-speed chain on 8-speed sprockets, as the transition from 8 to 9 was the major change from 3/32" roller width to 11/128". In most cases, the 7/8-speed sprockets were already thin enough, but there's bound to be an exception out there.
gbj_tester
If it's the same bcd could you swap chainrings?
9 speed has inner ring slightly over to the outside so the thinner chain can climb onto the big ring. This is the potential issue if you run a 8 speed chain in that it might rub when using the inner ring as 8 speed has slightly wider plates in its links, though irl it's normally fine.
If it does rub you could use a 9spd chain - I've done this with a 9spd fsa chainset friction shifting over a 7spd block with no issues. However if you're using indexed shifters I imagine it might make shifting at the back a bit tardy.