An .stl file would cary only the "hull" data, the overall combined appearance of all components.
Whilst that should be okay accordingto the TOS I do see that there would be a problem with mesh avatars, as basically it might be possible to use the .stl file of the naked body to import back into SL as a mesh.
As I don't have any mesh components I didn't thought about that. With a normal avatar body the problem would not exist as besically you would only have to note down all settings from the avatar editor and enter them on a new shape-file , a process completely legal, to get an identical body to work with.
And no-mod components you cannot dissassemble into their constituent components anyway, so you wouldn't get at the sub-parts to rebuild the item from the .stl file, but only a static .stl that'd leave out all transparents, would have static elements instead of flexibles, and so on, making the .stl file mostly pointless for reimport to Second Life, but perfectly suited for a 3D print.
I'm currently looking into programs capable of converting a series of pictures into a 3d model, which would allow me to generate a 3d printable file, but there's a lot of things yet unclear to me.
However it seems as if
http://www.arc3d.be/
and
http://meshlab.sourceforge.net/
might be solutions to my problem, working off plain pictures ( and thus also, hopefully, SL screenshots). Taking screenshots of the avatar in a white environment should do the trick for me I hope.