Yup, just like the forum sub-section you already created. People already have the means to find stakers/horses, that's not a problem. What's needed is a way to track all the important information that goes along with a stake and letting users develop a staking history. It seems like in the threads you've had a hard time getting people to put all of there thoughts in one place. I don't stake right now, but with a decent system in place it's something I would consider in the future. So, I'll try and outline what a good system would look like.
The best format for staking would look like the mp, but with more information. The information that is collected should be tracked and give each user a summary similar to the background check. Staking stats could even be included on the background check page instead of creating a separate area.
Make it the responsibility of the person looking for a stake to put in the basic details such as the game, the buy-in, and the terms. Like mp, they can have this as a private stake where they already know who is staking them OR they can have an open stake that anyone looking to stake can possibly accept. Like the way people negotiate trades right now, if the person looking to stake someone wants to modify the terms, the stakee can just repost the stake with the new terms, no big deal.
So the form for someone looking for a stake would give the tournament/buy-in/site/poker site sn (this can be auto-filled based off po info)/staking terms for the percentage (ex. 50/50, 60/40), and then have a box to check for stakeback, and another box to check for makeup. The person accepting the stake can then have a comment box to post any additional terms.
When the stake is complete, the stakee has to come back and complete another form that says what position they finished out of how many, and how much they won. Maybe you could even add a calculator that will automatically calculate how much they owe to their staker. That information can be collected to determine how well a stakee performs such as their average itm%, average roi%, etc. Include an option for the stakee to upload a screenshot as proof in case there are any discrepancies.
Once the stake is paid back, the staker should have an option to "complete" the stake confirming that the person paid back and that the stakee's results are accurate, and letting them leave feedback like mp. Feedback should look like feedback now, but possibly listed separately so that feedback doesn't get too confusing to look at.
Let me know if it looks like I'm missing something.