There are reports that grapefruit/grapefruit seed extract may impact hematocrit. However, donation/phlebotomy is the only certain method of making a significant impact on elevated Hct levels. Some members report varying degrees of success. I tend to think it's something like DIM and estradiol: if it made the sort of difference we all looked for, nobody would be using a prescription AI.
But there are those reports, and it certainly isn't going to hurt you. If you try it, let us know!