Hey all. I too recently picked up an iBooster from a 2019 Accord.
I'm familiar with the CAN systems on these Honda cars and have been working with them as part of comma.ai's openpilot since 2017; I'm a moderator on the comma discord. I got everything except the travel sensor rigged up this morning and looked at both can busses. With no other traffic, I am seeing just two individual frames being sent on each can bus.
F-CAN "B": 1BE and E7 are on the primary F-CAN which all the major ADAS modules are on. A basic definition of 1BE is already merged into opendbc listed as BRAKE_MODULE. 1BE is a status matrix of some kind (including the User Braking bit) while E7 from what I recall is maybe the pedal position both desired and actual.
F-CAN "A":1D5 again appears to be a status matrix (for faults?) while 686 is hopefully some kind of firmware/hardware version announcement (or perhaps VIN?); at first glance it seems to cycle through an index of some sort.
What remains to be seen is how independent this module can perform without the abs pump (actually) being present. The booster can request additional pressure assistance from the pump if something goes wrong, so the pressure signals will surely have to be faked on CAN amongst everything else. It's going to be a big project. I am also unsure if this module is paired to the immobilizer system like the abs pump is. Honda claims this module is "rewritable" so I'm hoping that purely means that it expects to see its paired VIN or maybe updatable firmware but hopefully nothing cryptographic.
I'll keep digging and post more later. Thanks for the info so far everyone.