The above technique is compliant with the license of Qt (LGPL). If Qt didn't want to allow it, they would have used a different license like the GPL.
There's nothing in the BSD license that prevents you from mixing in code. LGPL doesn't care if you link it against BSD code. Just that the LGPL code must stay under LGPL.
Also, LGPL is already problematic, because you cannot link statically with it.
Damn. I just wrote the whole top level comment to explain that you can link statically LGPL code and how to do it with details.
"If you statically link against an LGPLed library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application."