1.8 plugins will not work in 2.0 (1.9) without being updated.
The API has changed requiring all plugins to be updated before they will work in 2.0. The filter string is there to stop the download of 1.8 plugins which will break when installed.
See http://nathanw.net/2013/06/13/new-qgis-20-api/ and http://osgeo-org.1560.x6.nabble.com/SIP-API-Update-merged-Attention-plugin-authors-and-nightly-dev-users-td5059106.html#none
Edit: There is no workaround. The plugins just have to be updated.