Thank you Thomas. There are two reasons to load "structural metadata" into a Lua data page, to be imported with mw.loadData():
1. often the tl|Pg - which reads data page by mw.loadData() and converts a book page number/mark into a running link to djvu page - is called dozens, sometime hundreds of times by a single Page: this happens, i.e., into glossaries, indexes of people, indexes of lemmas; so, it's important IMHO that it's not server expensive;
2. I'm an absolute Lua beginner, and I'm searching for opportunities to test most interesting Lua features. ;-)
I'm going to expand the data page adding ns0 names of pages and subpages and ranges of pages which they transclude (all the needed data are wrapped into Index: wiki code in it.source); so I hope that tl|Pg link will point djvu page in nsIndex and nsPage, and will point to ns0 page/subpage when transcluded into ns0.