Would the Qiroll quick release help with your folding/seat post interference issue? It's not expensive, $24 USD.To answer my own question about Brompton compatibility.
It does fit in the rear but involves removal (or trimming) of the mud guards. Fold is affected because the seat post doesn't go all the way down. There is also this option for front mounting which I might explore at some point:
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Electric Brompton? DIY Friction Drive Solution
07Sep2019 Updates Upgrades for Turnigy Aerodrive SK3 6364 6374 190kV brushless motor See: https://www.thingiverse.com/thing:3849507 Instruction for wrapping SBR around the motor for best performance See...cults3d.com

At one point I installed my Qiroll on the front fork of a 26" bike that had an easy install unlike your Brompton. It seemed to work as well as it does on the rear wheel (caveat, I only did a couple short test rides).