The Java compiler and JDK have not been designed to work like this (using 1.3 JARs with the 1.5 compiler), so it's not certain that it's even possible, and your might get strange problems either at compile time or at runtime.
There's no official way to make this work and even if you manage to get it working, there's no guarantee that you'll get strange problems with the compiled classes.
If this is for a production code on a commercial, business project I would not take the risk to try something strange and unsupported like that.
Instead, I would first focus on finding ways to avoid the need to recompile the code so often. Is it really necessary to compile all those 14,000 source files so often? Is there a way to organize the sources in such a way that you only have to compile a small part if you make a change? Can you maybe setup a separate server which does the compiling at a moment when people don't have to wait for it (a nightly build, for example)?
(Ofcourse the compiler has "-source" and "-target" switches so that you can compile classes that are compatible with 1.3, but those unfortunately don't stop you from using Java 1.4+ API classes).