Hey there my friend, Segments have an almost "0" tolorance when it comes to transparency. a range of 15-25 or so grey in the alpha channel works but not well. It looks horrid in most cases, and I've tried nearly all other tonal values, with no good results. It seems to work on an all or none basis.
The best way I've found, is to use a combination of the two. Same as you would for a door or window segment. Use a frame of some sort to encase a semi-transparent entity for the glass. And save it as a segment. you can make the whole thing as big as a standard wall segment, or even larger. I use the FPSC segment developer tool for these types of segments, and they work like a charm.
2.5ghz QuadCore,4gb ram,1gb ATI graphics. Dual OS. WinXP, Win7.