I like an opposed Ride vs. Grapple.
(The following rules are entirely my invention.)
Treat it kind of like a grapple attack.
Entering the hostile mount's square provokes an AoO (like grapple step 1) unless you have the Mounted Combat feat. According to the (sadly untrustworthy) Rules of the Game articles, you need to be in the mount's square to mount up.
Mount as a move action, or make a Ride check DC 20 to mount as a free action (kind of like grapple step 2).
Now, since the mount is hostile, he gets an immediate grapple check, opposed by your Ride skill, to buck you out of the saddle (kind of like grapple step 3). If he wins, you're prone in an adjacent square. Use the usual Ride rules for falling from the saddle. (Choose a square randomly, or a mount with Int 3+ gets to choose which square you're in.) As usual with opposed rolls, ties go to the highest bonus, or reroll if same bonus and same total.
The rider must "pin" the hostile mount (rider's Ride check opposing the mount's grapple check) to get it under control. Every round, the rider must spend a move action and win an opposed Ride vs. grapple to keep the hostile mount under control. Failure means you're bucked from the saddle.
A hostile mount under a rider's control can only run, make move actions or make mounted charges. It won't attack anybody unless the rider also makes a Handle Animal check, per the Handle Animal rules.
So, under these rules, if you move in and mount up on a hostile mount, the mount gets a round of bucking (opposed grapple vs. your Ride skill) or biting (attack with natural weapon at -4, per grapple rules) to unseat you before you can get another attack action to get it under control.
That should be quite cinematic and lots of fun.