Using a LEGO software for Playmobil? I think maybe that's just the thing Richard had in mind.
I've used MLCAD in the past and should look up the current version and see what it's like. I haven't ever done object creation for it so would have to do some research. Heather at PlaymoDB might be another source we should talk to about this I know she has been involved in some LEGO projects in the past.
My ears are burning...
In my Lego days, I was heavily into LDraw, the open source Lego CAD system (ML-CAD is just one interface although the most popular), to the point of designing simple parts for it. It's very finicky work, but manageable, especially with a group working on it. K'NEX parts have been added to the library as well, I believe. I think basic System X and Steck parts wouldn't be too hard to add, since they have a lot of common elements: the sockets and flat Steck connectors, for example. Animals, plants, and klickies would be a lot harder because of the irregular curved surfaces, but not impossible. (The surfaces of Lego parts tend to be either flat or in smooth curves, which simplifies the defining.) I left Planet Lego just as the official Designer program was getting off the ground, so I'm not familiar with that at all; I have my doubts about the possibility of creating one's own parts for it, though.
One of the first things we need to tackle this project, which would also benefit PlaymoDB quite a bit, is a consistent naming scheme. My "Building" category is a mess. There needs to be a common way of describing a System X column, say, with its dimensions, how many sockets it has, on how many sides, how far apart they are, whether it's hollow or enclosed on any sides. Baseplates? How big, how thick, how many sockets, along what edges, any in the middle? It's complicated.
I have been thinking for some time about how to define part size to make it searchable, and I can see that it will be essential to any system involving physical definition of parts. Many of the German names of parts, which I've saved even when the part is named in English, include at least one dimension in millimeters, so I can squeeze them out to a separate field in order to show them. That's a start; collecting part measurements is a task that dwarfs the describing of klickies (which is pretty large!), but of course not all parts would benefit from being measured. I'll see how much data there is already, and get out the calipers for some others I own. It's a start.
I've been just thinking out loud here for a bit, but I'm glad the subject was brought up. Keep me in the loop, by all means, and I'll contribute as much as I can.