Thanks for the welcome, though I have actually been lurking for a while, I read a lot more than I post. There may well be a reason I couldn't find anyone posting about casting beans tube in....
If I understand your concern, it is that the beans above (and possibly alongside) the tube in the mold will float up and there won't be any coverage there. Maybe weighting them down with a screen or the other things people use would solve that?
My main concern had been that unlike in a drilled blank, the orientation of beans near the tube won't be as random. Since they can only start near the tube rather than have the tube get drilled through their middle.