You can already currently lean when in a resting sitting position. The tactical sitting stance really needs a lean feature. Also from the sitting position to side prone stance adjust instead of having to go to prone then to side prone stance adjust(ctrl+a/d)

You can see how when your character is sitting he should just be able to flop over to his side (especially sense there is no side stance adjust(ctrl+a/d) function for sitting)
it can take way to long to get into this position when under fire by cycling though them.