FDM-type partial fill isn’t such a great idea with SLA printing, because there will still be resin in the uncured internal sections which doesn’t save any resin and can make a mess if the parts fractures.
Formlabs would have to guarantee that no pockets are created, and that the bottom is open such that the resin can escape, which will cause cupping, forcing you to create vent holes. All of that without speaking about the fact that emptying and post-curing will be difficult.
When trying to save resin I would go the route of simply hollowing the part, which meshmixer does very well on existing STL files. You’d still have to add a venting hole depending on print orientation (to prevent cupping) and you might end up with supports inside the part, which ou may choose to remove or leave in place… but in all cases it will be easier to clean the uncured resin from a “simple hollowing” than a partial fill à la FDM.