Just to add some inspiration to the topic... I'm thinking of doing this "hack". While it doesn't use Shimano's levers/brakes, it uses the TRP Hylex hydraulic brakes which are quite comfortable, ergonomic, and very nice stoppers. It also saves a bit of $ as you don't need to buy the levers, which are often the spendiest part of the drivetrain.
https://www.instagram.com/p/BVZwoRyBIWI/?hl=en
Croz
eTap/Di2 Hacking
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