I used to donate plasma twice a week every week for about a year while I was in college. I exercised a lot but didnt really watch what I ate- and it didnt cause any problems as far as my physical activity went (Major in dance plus 60-90 minutes at the gym and teaching dance as my job).
Now- I weigh less, excersize about the same amount- but count calories and eat a lot less. I would love to get back to donating plasma but don't know if that's okay to do while in a calorie deficit (and training for running races)
I know where I used to go they did a finger prick and checked iron levels etc to make sure you were fit enough to donate. I personally no longer try because 3 times out of 4 my iron levels were too low and I couldn't donate in case something went wrong and they couldn't return the red cells.
I would think that you lab would be the same, testing for deficiencies that may cause problems. So I'd say give it a shot and see how it goes