Second time round doing reverse foot, I realize now that it wasn't completely working when I did it last semester.
I'd only the "step" working, while it caused all other attributes of the footManip to stop working as the step overrides the attributes set up via connection editor. After sleeping over it, the next morning, Eureka! I got it working. Why, yes, I should take the driver and driven both from the attributes in footManip for the "step".

Ok, so here, I just put down the settings I used for Anna... so I can refer to here again next time!

Reverse Foot Joints:
heelRev
-- toesRev
---- ballRev
------ ankleRev

Parenting IKs to Reverse Foot Joints
ankleRev
-- legIK

ballRev
-- ankleIK

toesRev
-- toesIK

footManip custom attrib limits (adapt from [1]):
ballRot         -10 to 45 ([1] has 30 which i think is too low)
heelRotX     -90 to 90
heelRotY     -90 to 90
heelRotZ     -45 to 45
toesRotX     -90 to 90
toesRotY     -90 to 90
toesRotZ     -45 to 45


Adapted mostly from [1] in setting up reverse foot:
  1. IKs: legIK, ankleIK, toesIK and parenting to reverse foot joints (ankleRev, ballRev, toesRev respectively)
  2. Reverse foot joints steps
  3. footManip with custom attributes for x,y,z rotation of heelRev, toesRev and z rotation for ballRev; footManip settings same except for ballRot increase to 45 (rather than 30 as I find 30 to be too low)
Adapted from [2]:
  1. Reverse foot joints - snap toesRev to toesEnd
  2. Added "step" adapted from "roll" to footManip
  3. Used set driven key for "step" (used -30 rather than 20 for Heel Rot Z see note 1)

Note
  1. Aligned all z-axis on local rotation axes of reverse foot joint - I flipped heelRev z axis by 180; so in following [2] for step (roll in [2]) I set Heel Rot Z to -30 (in [2] it is 20)
  2. Animation in and out tangents - clamped; Check footManip (compare with graph editor for heelControl)
  3. Set Driven Key: driver: footManip-step driver; driven: footManip-Heel Rot Z, Ball Rot, Toes Rot Z (do NOT use the reverse foot joints heelRev, toesRev, ballRev in drive - this will mess up the connection of footManip custom attribs Heel Rot X/Y/Z, Ball Rot and Toes Rot X/Y/Z to the reverse joints rotation attribs

References:
  1. Learning Autodesk Maya 2009 - The Modeling & Animation Handbook (Lesson 12)
  2. Learning Autodesk Maya 2008 - Foundation (Lesson 12 p274-282)