I pronounce 100.1 “one hundred point one” (which I believe is how most people in the UK say it).
In fact the “a” may turn up among the positive integers much sooner than 1000, or even 101. In the past you could say “one and twenty”, “two and twenty” etc for 21, 22, etc (cf Jeremiah 52:1 of the King James Version of the Bible: “Zedekiah was one and twenty years old when he began to reign …”). This is now archaic in English, but is still used in German and Dutch: 21 is einundzwanzig in German and eenentwintig in Dutch. So, I suppose, depending on which style of English you speak (as well as your views on the numerical integrity of the word “and”), the letter “a” may appear as early as 21, or not until 1000.