does anyone know of a lint program that (can easily be modified to) will accept '51 extensions and preferably test them for validity.

Yep. I do.

A few years ago we created configuration files for all Keil compilers for PC-Lint from Gimpel Software. The output from PC-Lint is really nice.

The configuration files include information so that PC-Lint knows about all the C51 compiler extensions and doesn't give superfluous warnings and errors.

You can find out more here. http://www.keil.com/pclint/

The uVision2 IDE will run PC-Lint on a single file or on your whole project (which is the recommended procedure).

It is definitely worth the money (approx $250).

Jon