Hi Michael,
DITA DTD does not support baseline offset for images w/o making a spesialisation for ,say, fmimage from dita image-element(i've done it in few projects). In DITA 1.1 it is possible to make attribute specialisation to store the offset value.
If you can survive with a static offset value, rw-rules can be used to override the default 2pt offset.
Martti