Asperger said:
You mean the possibility of a one-sided (half) open borders agreement: the other may enter your land (only missionares), you not their land. Interesting, would the AI have arguments not to sign such an agreement?
I'm sure Isabella would agree to such an arrangement.

Yes, that was roughly my idea. In fact, who would reject an offer like that? Such an invitation, if accepted, should affect your attitude score with that civ as well.
Here's an amusing story, by the way. The other day, I played a game where Isabella decided to exterminate me because I (Elizabeth) was the evil founder of Hinduism, which had been accepted as the official state religion by almost all the other civs, whereas she was a Buddhist. (Those who were not Hinduists were Confucians.) Her soldiers kept attacking my walled cities, which were garrisoned with axemen (later upgraded to macemen), spearmen (later upgraded to pikemen) and archers (later upgraded to bowmen); my garrison units became absolutely deadly after twenty failed attacks or so. I also had a few chariots, and later on a mounted archer or two, for fending off attempts to move further into my territory than the two border cities. And yet, the little lunatic kept attacking my cities, when she didn't send out expeditions that tried to pillage my improvements. My sentry units became quite experienced too... However, my lands around those two cities were somewhat underdeveloped, as you may imagine, Still, I had lots of gems and gold and sugar which I traded with other civs for other luxuries, so my population never got tired of war. (The whip helped too, of course.) She did not advance far enough up the tech tree to build catapults or macemen, but I was unable to churn out more troops than I needed to save my cities and keep her from pillaging my land. She had lots of cities that kept producing more swordsmen and chariots and swordsmen and swordsmen and swordsmen and an occasional axeman. I got Great Generals and used them to build a war academy and install a military specialist in one city, and after having built some units using them I started to build the Heroic Epic - I should probably have postponed that. If I had had iron and been able to build swordsmen, she would have been dead meat before her numbers started to tell; but I was boxed in (playing at a fractal map), so I had no iron and never had more than four cities, all of which were kept busy producing military units. Frustratingly, her iron mine was just a couple of tiles from our common border. (I could not even spare the units needed to capture and pillage it.) The only Wonder I managed to build was Stonehenge, and that was before Isabella decided to exterminate me. That meant that a culture bomb (so the her iron moved to my side of the border) was impossible; all I got was prophets. (Build a theatre? You must be kidding. Build... more... units...)
When we started fighting, I was number one in score and she was somewhere down the middle; after thousands of years of incessant warfare, we took turns being at the absolute bottom in scoring. I had to keep fighting because I needed that iron desperately. I also needed to expand, of course. Nobody else wanted to join a war against Isabella, who was militarily strong and had many cities. They would very likely have joined in against me if we had not shared the same religion. I could probably have sued for peace, but what then? Still boxed in with four cities and way behind in research. By the 14th century, I had discovered Guilds and could have built knights to roll over her with, joined together with my catapults, pikemen and macemen - if I had only had iron... As it was, I could keep her at bay but could never have made it to her border city against her her superior numbers.
I believe I would have managed to capture that city (her border city with the iron) from her in the end, but at this stage I was so far behind the other civs that it wasn't worthwhile to keep playing. It was an entertaining game, though. An interesting thought: even though nobody wanted to join me in a war against Isabella, they must have refused to trade technologies with her since her units were so outdated. The other civs were all happy to trade technologies with me, though.