Based off the beta, I believe it will be possible to use linear with hip. I will still be using a curve for ADS though, purely due to Aim Assist.
You discussed building muscle memory, etc. IMO, this is only important for HIP because HIP doesn't have the massive slow down effects like ADS does for Aim Assist. You can only build muscle memory to something that can be repeated on a consistent basis. I would argue that AA when combined with net code, lag, etc. pretty much destroys linear movement and therefore doesn't apply to ADS aiming. For me, all that matters is how my ADS aim feels and respond's, when engaging AA at different ranges, speeds, etc.
Many have said (including Admin) that curves eliminate linear movement, and I don't disagree, it is true! However AA also eliminates linear movement, or I should say "consistent movement" by drastically slowing your movement when your cross hairs are close to a target. So I ask, when Aiming down the sights. What really feels closer to linear or "consistant movevment" while engaging a target? No curve that slows drastically near target, or a curve that helps maintain velocity when hitting the AA bubble?