Update: creating the dictionaries with Mobipocket Creator Publisher Edition gave better and more consistent results than using the command line mobigen.exe.
Get it from:
Create new publication
----create presonal dictionary-glosary
important: specify the language, and select the input-output languages at the book-setting tabs, without this, instant lookup won't work. (Also I got the best results with unicode encoding.)
drop all the numbered .html files created by the tab2opf.py (leave they .opf file alone)
click on build