Is it possible to export a skeleton that has been fit to the actors T-pose as the first frame?

Example:
I have fit the skeleton to the actor T-pose in frame 48. Then I create a 'take' that is at frame 1000. When I export to BVH for the 'take', can I have iPi export frame 48 as the first frame in the BVH file?

There is the checkbox names "Export T-pose in first frame" on the "Export" tab. It will set the first frame to a "real" T-pose of the skeleton, in which all bones rotations are zero.
If you need the exact pose of the skeleton matching the actor in specific frame, then you can export this frame individually and manully insert in the beginning of your take BVH. As BVH is a text format, its easy to do. Do not forget to increment the frame count value.