Hi, Neoflash:
You will not get infinite V. Basically, when you drive a differential pair at the the middle of a TLN, you are actually creating some discontinuties into the structure. You will have common mode around it. The Zin at the feed point will not go to infinity while it will be a big value. Therefore, if you try to inject some current into it, you will be able to inject some slight current into it while it is very difficult to build it up. I think the situation is simialr to you have a tank of water and its cover is sealed very well. Then, you try to push the cover down. Practially, you will be able to push down very slightly and the pressure you put on it will not go to infinity. Howver, it will be very difficult for you to push it down significantly. It is basically the same situation.
Best regards,