I have had this conversation with a couple of people, and here is what we came up with:
Most of the Ride skill rules only cover riding a mount you do control. Mounted Combat's ability to negate hits cannot be used unless you are controlling the mount. Neither can any Ride skill check to increase the mounts movement or Jump results.
When riding a humanoid, unless its INT is 2 or less or is somehow macigally dominated, you can never treat it as under your control. Mounts not under your control act on thier own initiative. Ride checks are required to remain mounted as per the usual rules.
Generally the riding character will be riding without a saddle, which increases the DC of the check. Unless your party is crazy enough to purchase custom made saddles for themselves. A backpack with holes cut in it isn't enough to count as a saddle....
That means the riding character must Ready attacks to strike targets along the ridden characters movement path. Obviously it easier to use ranged attacks
The riding character
can use the Ride skill option of gaining Cover.
In short, the Rider needs to/can:
Stay Mounted DC 5 {if mount is in melee combat or either of you take damage }
Guide with Knees {if you want 2 hands free} DC 5 {this is actually a 'HANG ON with knees' check ;p }
Cover DC 15 {as a non-action, gain cover behind your mount. You cannot attack or cast spells while gaining cover this way }
Without a proper saddle, the DC of these checks increase by +5
The normal Ride synergies {Handle Animal and Animal Affinity} only appy if the mount is of Animal or Magical Beast type.
Using this tactic generally means they get one attack or spell per round, benefit from cover and are not limited by thier personal movement rates.
I lost the thread, but somewhere recently there were some good rules on riding unwilling creatures, complete with bringing a mount under control. IIRC you essentially had to Pin the mount.... I know I am missing something there tho...
Anyway, thats what I use. I do have a Halfing Outrider in the group and am quite amazed that she hasn't hopped on the Half-Orc's shoulders yet.