I believe I sent you an email about this as well, but I'll answer it here just for completeness.
We would love to be able to use the 250000 baud rate, but finding cross-platform support for it has been quite a challenge. We actually re-evaluated 4 different libraries over the weekend to see if any of them could offer that support, but not a single one did. We did find one that would work with a single OS, but we're trying to avoid OS-specific code as much as possible. From discussions with some of the other developers, it sounds like support might be added in the future, but the drivers the different libraries use don't support it right now. So short answer is, we will keep investigating it, and if we can find a simple way to do it on all platforms, we'll make it happen.
And the idea of setting the baud rate arbitrarily based on a textbox is probably not feasible. You really need specific driver support for different timing and transmission rates and it isn't an arbitrary choice. More importantly, the microcontrollers we communicate with also have fixed clock rates which mean that specific frequencies actually give better performance. Have a look at some of the ATmega datasheets and you'll see what I mean.