Nice to hear from you Dave from Canada.
I too would like a feature like that in the editor. There were some attempts at that way back in the day. One of the early issues was a lack of metadata about the tiles that classified them as specific types. The tile set files are divided by terrain type, but don't distinguish between say a regular flat tile, and a ridge tile. There was some tile group data found appended to map files. It's not read by the game, but could be useful for an editor. It was extracted and I believe became part of the editor, but it seemed to be aimed more at multiple tile doodads, like volcanoes, and large meteor craters. Some of them though were just a grouping of related tiles, like rubble, or small meteor craters. Again, no metadata to describe if the group should be placed as a whole, or an individual tile selected from the group.
I think what might be done is to just find the large group of flat tiles for a given terrain type, and randomly select from that for an area placement. The problem does seem quite doable.
I should also point out this has nothing to do with the SDK used for building levels. The map editor is a different thing entirely. It's also not in the SVN, although, it probably should be. I don't think the project was updated since the SVN was created. There are different versions, but the one I remember was partly written in C++, and partly written in VB, and the source for the different parts may have been released at different times. You can search the forum for "mapper" or "map editor" to find relevant threads, including a toy tool to view the tile groups. I'd try to point out specific pages, but I'm on an unreliable internet connection and the pages don't seem to be loading. Perhaps someone else can summarize the relevant pages?