My problem with that is that it's simply 2 games, one of which is influenced by the other, but nonetheless, 2 games. I think it makes more sense to have the players on the board be the players in the bourse. This encourages profitable deals to be made etc, and in the end it is all about the money, nothing else.
This actually changes the boardgame because people try to manipulate the market in their favour. The problem with just that is that people could decide to effectively suicide to manipulate the market in a very unexpected way to give them a big edge in the bourse.
While I don't mind that, there has to be some incentive to actually make your country prosper to win the bourse (which is the goal like I said, what happens to your country only influences your money, other than that your country doesn't matter in the end, by my rules), otherwise the boardgame tends to become ridiculous, as has been proven I'd say. That's why I'm suggesting an income based on the amount of SCs your country has. This gives you incentive to make your country strong and your currency valuable, since more valuable currency means your income increases too.
The SCs influencing the bourse is not needed I suppose, but I still think that would give more intuitivity, and more incentive to make your own country prosper, because then the income you get is worth even more (since you get, say, 10 denarii per phase (spring/fall) per SC, and that 10 denarii is worth more if you have more SCs as well), and, well, just more intuitive.
But I don't need the SCs directly influencing value if people don't like that, maybe I see that wrong because I haven't played yet anyway, but without income or some other incentive for your country, I don't want to play it because it are 2 games with a connection that works only one way. I think the bourse should influence the board game, in a way that doesn't inherently reward the occassional suicide in the bourse.
Income is the best way to do this I think. I simply refuse to play without some incentive for the board player to both be succesful and influence the bourse. That's the whole point for me.
I also think it's realistic; countries can trade all they want, but only in very extreme cases would they consider losing ground for a big reward in cash. My income-idea represents this quite well I'd say.