Honestly, the way RFCE++ did it was fine, although it in itself seemed too gamey/unclear on whether the AI would accept or not (e.g., as Prussia, Germany would never let me get Berlin through that minor PU).
Wait, there already is a 'Personal Union' mechanic in the ++ version? What are the specifics? (I'm waiting for a stable version, so I've only played 1.0).
New proposal!
The big idea for today is: post-combat ransoming!
Basically, given the importance of nobility in medieval warfare, those nobles often found themselves captured by the enemy after a military defeat, and would only be able to return to their estates after being ransomed back.
To reflect this, I propose that after each military victory, there should be a certain percent chance of receiving a certain amount of

. Specifically:
After defeating a infantry unit (melee or archer) or light cavalry, there should be:
a 5% chance of receiving between 1-5

, and
a 1% chance of receiving 10

.
After defeating a heavy cavalry unit (lancer, knight, etc.), there should be:
a 50% chance of receiving 5

,
a 25% chance of receiving 6-10

, and
a 25% chance of receiving 11-25 gold.
(In other words, you would always receive a ransom of at least 5 gold after defeating a knight, and the value of that ransom would go up depending on the value of your captives.)
Forts and city walls make all units in that tile immune to ransom effects (so defeating a knight that's defending a city would not give you a ransom, since you wouldn't have captured them). We might make an exception for the final unit defeated as the fort or city is captured.
This ransom mechanic could be an isolated effect on the winning civ (the money appears out of nowhere) or ransoms could be transfers from one civ to the other (the money appears in your treasury while disappearing from your enemy's). I prefer the second one, but I leave that to others to decide
If we go with the second option (ransoms as transfer payments), we might also give a corresponding minor benefit to the civ that loses funds. Specifically, that on paying a ransom, that civ receives a number of

in production equal to the amount of

lost. This production would go toward building a new knight/heavy cavalry unit in their capital city.