Has anyone seen such a thing? I know that Mountain Hardwear used to do a Conduit SL bivvy bag with said zip but it seems they're long gone, just wondering if there's anything else.

Not much about,

Big Agnes 3Wire https://www.bigagnes.com/Products/Detai ... reBivySack

Or try and locate a Rab Assault, they had a long zip as I remember

Already own an Assault DNF ... it's a lovely thing but wanted something a bit lighter, I recall the Assault is nearly 800g

Can't you make one? Maybe talk to PHD, their lightweight sleeping bag cover is a nice bit of kit, my extra long, extra wide one comes in at 190g. Maybe the could add a zip?