661 do a really lightweight thing, I can’t remember the name. Sub Gear? Unconvinced it’s worth a damn tbh, it’s pretty vestigal padding.
Chests are just plain hard to protect, chest injuries tend to be more about momentum than about blunt force- ie, it’s your body that does the damage, not the thing you hit, it just stops you. So I suspect anything that’s going to be useful, is going to be pretty substantial.
But I know people who happily ride XC in full dh pressure suits so it’s doable. I don’t enjoy pedalling in my dh jousting suit but I think if it was neccesary to keep riding, I would. Or rather I’d get a better vented one!
Roost guards aren’t really suitable, they’re for stopping small hard things that get flung at you so they tend not to be designed for impact absorbtion. Plate armour, not cushion.