If I add slf4j-api-1.4.3 and slf4j-log4j12-1.4.3 to the Eclipse RCP target platform, I'm not able to export the application due to cycle dependency (API depends on impl package to provide implementation and impl depends on API to define API).

Fix:
Just add this line to every MANIFEST.MF of the JAR providing org.slf4j.impl:

Fragment-Host: slf4j.api

Then all works nicely.