On this article, we can read that the Cure was planning to play shows in Bulgaria, Poland and USSR during the Prayer Tour. Did you know this? I guess these shows never happened because the politic situation was not that stable in these countries in 1989.
This article is pretty interesting, there's a list at the end of the planned French shows. Only one Parisian show on Ile St Germain in open air was planned (i read that the promoter could not secure this one and that the Cure finally had to go to Bercy for few nights) and the Caen show was not planned, there was an Orleans show planned instead. And no mention of the cancelled Boulogne sur Mer show.
I don't remember very well what I read in 1989... I have this magazine and others but... I have read at the time Robert wanted to visit some eastern european countries for the first time, but i don't remember exactly which ones were really planned and how things changed. Sorry, I can't help...