Ok. Thank you for your time and patience.
Even though - sorry, just last try. I am actually not asking for Calibre not to point guide item type="cover" to image file. Calibre probably needs it in some situations. (Even though I can't find example where Calibre actually
do that. What I can see are guide items type="cover" pointing to html... That's not so important.)
What am I actually asking for is
just 
a minor adjustment of "Go to..." menu creating algoritm... So
just in case guide item type="cover" is pointing to html (for whatever reason),
than the creating of the special menu item "Title page / Cover page" will be abandoned, and
instead item type="cover" will be processed in more standard way...
(Also: What is the use of the special menu item "Title page"? Isn't it always pointing into the same page as menu item "Book start"?)