We have our meetups on the 10th of each month, weekdays at 18.00 o'clock, otherwise per arrangement. The place changes, because Berlin is big. You can always find details in the German forum. Additional meetups are held if we have visitors.
In case anybody wonders whether we go to a BookCrossing wedding... no, the meet-up is in Wedding, the Berlin neighbourhood (pronounced like vedding), notorious in the 18th century for prostitution and gambling, notorious in the 21th century for its Bookcrossers. https://en.wikipedia.org/wiki/Wedding_(Berlin)
I am not from Berlin, so I do not know the best places. But here is a bookcrossing guide for Berlin https://www.bookcrossing.com/---/9769774 Unfortunately it is only in german but you can find lists of all the OBCZs in there.
I never heard of any such rules. I just don't usually do it inside museums and palaces - too many guards. (Exception: The Museum of Technology! Once I overheard two guards there, one had found my book and wanted to give it to the other one to bring it to Lost&Found - and guard no. 2 had a quick look at the book and then explained BC to guard no. 1, and that he should either keep the book or leave it somewhere in the museum for visitors to take! So I followed guard no. 1 while he read the blurb, and then watched him leave the book at some prominent place next to the huge old steam engines - wasn't his taste, but he got the idea.)