From the license point of view:
Is it possible to use a plugin written under "LGPLv3+" in a program protected by "GPLv2"?
The plugin is built as a ".so" library and at runtime linked to the application.
This answer is predicated on the assumption that dynamic linking does make a derivative work for copyright purposes, which is not a settled issue (pro, contra). That said, in my opinion, you may not do this. Any additional code used in a GPLv2 program falls under GPLv2 s2b, which says that
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License
LGPLv3 permits on-the-fly relicensing under GPLv3 (more accurately, it is GPLv3, with a set of optional extra permissions), but that's no help, because the FSF say that "GPLv2 is, by itself, not compatible with GPLv3".
The FSF also directly address your point, and say that "LGPLv3 is not compatible with GPLv2 by itself".
If the original work had been GPLv2+, you could have taken it under GPLv3, and (as the FSF agree) there would have been no problem. But it's not, so you can't. Sorry.