This doesn't answer the original question, but I've wondered about surfactants that enhance or at least don't hurt biological activity for foliar sprays. Most common surfactants are intended to help reduce/destroy bio-activity, so we're looking at the issue from the other end (as usual).
Musing without a lot of pre-thought, I've wondered how processed skim milk might work. The active surfactant agent(s) most likely would be the phosphoproteins, which would require some denaturing of the milk protein to unwind the casein to a simpler state. In coffee shops, they do that by steaming the milk, but I've wondered if an enzyme chop would do something similar. I was thinking of maybe a bromelin or papain prep to keep the cost low. There might be issues with the milk interacting with salts in the foliar spray and maybe with residual enzyme activity either with the spray components or on the plants. I don't know whether the quantity of milk that would be required (if it even would work at all) would reduce milk's effectiveness against mildew in the application area.
On the other side of the phospho surfactant idea is lecithin, which is a phospholipid. I can see how it might be a fair candidate, and it would seem to me to be fairly benign. Again, I suppose it could have some issues with salts in the foliar spray solution. There probably is quite a difference between water and the chemistry of a foliar spray solution.
As an aside, I'm not sure how damaging propylene glycol would be on the bio-activity. The fact that it is (or was) in diet Dr. Pepper doesn't give me much comfort. One can get to/toward PG from coconut oil, but it isn't a far jump from coconut oil to soap/detergent either, so that probably moves too far toward an undesirable biocide/biostat effect. I assume that straight coconut oil doesn't perform as well as yucca oil, or else folks wouldn't be using the yucca.
Personally, I prefer the sticky approach of sugars to the spreading effect of surfactants. I know that a California food producer developed a delivery mechanism that generates an electrostatic charge to drastically reduce the required application quantity (of synthetic pesticides, in their particular case), and that sounds like a good overall approach. It apparently took a lot of R&D though. Again, the soup of a good foliar feed spray is a far cry from the homogeneous solution that that company was using in terms of how easy it would be to impart a charge to the solution.