There is a beach script included with mod 11. It just isn't assigned to anything because I didn't feel it was worth wasting a spot in the palette for. I did actually leave 3 empty slots in the "official" palette ( FC,FD,FE) but the one I distributed is something I was messing around with and I added some extra dirt heights in those slots.
BTW Vip, group 5 in mod 11 is desert. Graveyard isn't actually assigned; I only did it on someone's request but didn't consider it worth using a palette spot either. Not relevant for you maybe, but relevant for him since he is using mod 11.
The way I do it though is to edit the area in the items.txt file to populate different areas of the same terrain with different statics. See
http://www.ryandor.com/forum/viewtopic.php?t=1115 for more details. In this case what you would do is paint desert and beach alike with the same group 5 sand. Then before you compile you have to figure out which areas you want to be desert and which sand and edit items.txt and supporting scripts accordingly. It is a lot more work but also gives a lot more flexibility.