I am also on one such project. I have a small idea on which I would like to get some inputs from members. In a typical data logging setup, there are some channels which are relatively slow as compared to rest - for instance a temperature channel where the values almost remian constant for a large number of samples.
It will be exteremly confusing if you makes some data log in one way and some in another. Why not make all data storage a byte count followed by a value. Even fast changing data should sometimes achieve a count. This, of course, means that after 256 identical readings of a temp you will store it again, but is it really worth it to makes extremely complicated code to save a few bytes, after all you can buy 2Mbytes of flash for ~$3 (see above).

Erik