What should you consider when playing an Open-Tuning where more open strings will be played and the overall sound will be brighter than the equivalent notes in standard tuning?
In my opinion: Nothing. That's an effect of the open tuning, and you also often have longer sustains on the open strings. The chords themselves sound different, with different emphasis on different strings.
It's neither better nor worse, just different.
In each case the guitar, mic and amp (if any) will have a big impact as well, so the only thing you need to consider is if the end result sounds good to you or not.
You should consider the balance with other instruments. If your extra brightness starts to interfere with another instrument, then you'll likely want to EQ one of them down a few dB in that range to prevent muddiness.
Consider using a spectrometer on two separate recordings, one of an open tuning and one with the same notes in standard tuning. See how different they are and in what range, so that you know where to EQ if you need to.
You should also consider how your recording equipment (mics, preamps, maybe guitar amps) are going to respond to the extra high frequencies. I don't imagine it'd make too much of a difference, at least not enough to use different equipment, but there's no better way to find out than to try it and see if you like the sound.
Agreed you should consider the balance of the other instruments but if you really want to go lenghts to do a exact EQ based on the note, you will have to do some maths.
I do understand what you are saying. I do notice a lot of guitars reproduce some strings louder than others and the effect becomes far more noticeable with open. Fixing your amp settings or (if it's an electric guitar) maybe readjust the pickup should be your first focus.
If you need to go "whatever works" on this, try this: (FH/FL) = 2^(x/12)
say, you want to find out A3. Then FH will be a reference for your higher note (A4=440 ie). FL will be your result, the rest is 2^(12/12) cause x=number of semitones.
Now, seriously, do you want to have to do this in the middle of a mixing session? :P
Having said that, I had to use it on a project handed in to me to mix. But did it before hand, got it out of the way. That was my corrective EQ. Nowadays I've been trying to be as careful as I can to avoid EQ'ing at all. But this damn economy crunch is making the small rooms thrive and the big, nice rooms go bankrupt arghh!