Two other ideas.

1. Drill holes in the lug, thread them, then use grubscrews to hold an aluminum post. If this doesn't work (quite likely there is not enough thickness in the lug), then try . . .

2. Use an aluminum seatpost with thick walls, drill holes in either side post and lug, thread the holes in the post, and use short bolts to secure the post in the lug. If this doesn't work, then try . . .

3. The through-bolt thing.