I've heard of Lightwerkz before in NJ since they do so much work on e46/e90 and other makes but I came across their recent thread for a 135i which uses a semi-permanent sealant from factory (sounds familiar??)
http://www.e90post.com/forums/showthread.php?t=814850 (see post 17)
So I reached out to him and he said he can definitely do the Z4 lights as well. I'll report back in a few...