Thanks everyone for your suggestions!
Roberta - great idea, I ordered a waterproof saddle cover. I did, however, put my C17 back on for now, and i'm thinking I'll just use that saddle with padded shorts for rest of the summer. I'm an hour north of Toronto, 8 of 12 months of the year are relatively cool (I ride early) and I can go with my preferred chamois-less set up, which has been great until recent summer weather (last few years I was on C17; haven't used a leather saddle for many years, and back then I'm assuming the quality was very good.
Jim - I am concerned that applying anything to the top will soften it. A thin layer underneath was more about rain protection, in the small chance I'm caught out in the rain and it doesn't have any treatment. Question for you - you use Obenaufs, is that top and bottom? If it's also to the top, does it somehow resist significant sagging in the sit bone area if it's sweltering outside and your sweating? That's definitely not the case with proofide in my experience.
To be honest, I don't understand how anyone can ride a modern day Brooks on hot days and not prematurely destroy the saddle. I rode this morning, it was 21C feels like 25C and did feel very humid (much better than 35C feels like 45C in the last week). 1.5hr ride and I was done, because the saddle took on a look like I had used it for the last 20 years, seriously, it was that bad. And the craters in the sit bone areas mean the part in the middle is uncomfortable. I'm letting it dry out and then I'll concerning tightening it a 1/4 turn. Sigh.