JP Routens (JPR), a French company, made seatposts with two bolts that microadjust nicely by pivoting (rocking) the clamp on a curved surface on top of the post so you can get it exactly to the angle you need.
I have it on my Peugeot PSV, and has given me no problems since I bought it installed as OE on the bike in 84.
Look under "JPR" in ebay. they had a good number in differing sizes (27.2, 26.8, 26.4..etc.) for auction earlier this year.
Chombi