I do not think that I am in a position to provide any concrete suggestion on how to analyze the irregular nucleic acid structures from your MD simulation. Nevertheless, it would seem to push 3DNA (or any analyzing tool for that matter) too much trying to calculate base-pair parameters for a flipping out base or step parameters for a non-double helical region.
You could, however, calculate all the backbone and chi torsion angles, and sugar conformational parameters (with find_pair -s option). You can also generate a blocview image (blocview -d my.pdb) and display the block image with PyMOL (pymol t.r3d).
As with any software tool, the issue is not just about calculating some numbers, but more importantly on understanding what the numbers really mean.
HTH,
Xiang-Jun