Just an update, I got my Krown done. They recommended me to do it in August because it's warmer and the oil is more viscous (which makes sense). Luckily this weekend was quite warm so it wasn't that bad to do.
Got it done at the Krown dealership at 33 Parliament St. I told them no holes to be drilled so they just sprayed the bottom. The holes are for inside the doors and they just fill the inside of the doors with oil.
Logically though, I wouldn't worry too much about this part because BMW's paint will be enough rust protection and I won't get exposed metal through rock chips in this area (and if I'm really paranoid I'll just flush this part with some water though the drainage holes using a supersoaker or something).
I was advised not to wash the underside of the car at all from now on and next year, they'll apply a foam which will wash off the oil and all the salt. Apparently Krown is supposed to be put on once a year. I'm thinking I'll wash the underside when I change back to summer tires because it's better than the oil trapping moisture which in my opinion is actually worse for corrosion.