Hi Ronald
You could create an empty (dummy) text object in the datawindow trailer using an expression like if( condition, y1 , y2) for the y property of the text object. y1 & y2 are integers representing the position you want the text control to be shown show that the band has the correct height. Setting autosize height in the trailer band will force the band to resize acorrding to text control's y position.(Provided that the rest of the objects in the band show up earlier)
I hope that solves your problem
Regards
Konstantinos