When i had my transplant i weighed 42 kilo's and i'm 1.54m. My tx doc never pushed me to gain weight, he said this would be fine. And it prooved to be. I do know a girl who had to gain weight before they would list her, but i don't know what her bmi was. I did loose some weight after transplant but not that much and i gained it back again soon, even with less nightfeeds and i could stop that alltogether when my weight was up again. In the hospital they wanted me to quit that even sooner but i knew i didn't eat enough for that (hospital food :S) I can really notice how much energy even only breathing took with my old lungs.
I think my BMI was about 18. After my tx the surgeon who did the transplant said my weight was good for them. I didn't have much fat at the site of the incision so they didn't have any problems there. And they could close me up very nicely, took extra care of it cause they could see it could become a nice scar. Everyone who saw my scar, even a few days after my transplant, said it looked so nice and it healed very good.
I don't know what the ideal BMI is. I would guess they wouldn't say i'd have had to loose weight if i was a few kilo's more haha. I think they'd be just as happy.