I have the opposite problem. Just bought a secondhand Trek Madone SL7 52cm frame. I'm 5ft 8 and TOO SHORT for the lowest seat post position! The mast won't physically go any lower within the seatpost, Also, the lower bolt you tighten it up with is nearly buried below the access slot. I need another cm or two lower but if this were achievalble, the lower bolt would be inaccessible. When I tried the bike, the saddle was pointing slightly upwards so its rear end was a tad llower - it felt OK. For longer rides, I need the saddle more horizontal.
Something odd here as according to Trek's size chart, I should be slightly too tall. I prefer the smaller frame size because of reach. The seat mast is not very long, less than 30cm.
Any ideas how I can get round this?