Dear Charles Nichols,
Your question induced me to google "charles xavier" - - and it turned out that there were too many hits to follow, even after I subtracted "x-men" and scifi.
But it seems that the name belongs to someone who might have been of interest to Nabokov, Charles Xavier Thomas (1785 - 1870). On this web page:http://www.thocp.net/hardware/aritmometer.htm you can see his calculating machine, which predates the Ada Lovelace - Burbage machine, which VN does refer to in Ada. On the other hand, I don't recall anything in Pale Fire on this subject, so this is probably coincidental.
As to the X-men, though, there I can't help you.
Carolyn
p.s. the name Xavier on its own seems to be of Arabic origin meaning splendid or bright.