[Proj] Changing Radius of the Earth?

[Fischer, Robert P. (GISS-6110)[COLUMBIA UNIVERSITY]]

How do I change the radius of the (spherical) Earth in proj.4?  I'm using the proj.4 string:

    " +proj=laea +lon_0=-40.000000 +lat_0=74.000000 +x_0=1000000 +y_0=1700000 +ellps=sphere"

..and have discovered that proj.4 assumes a radius of exactly 6370997m (from pj_ellps.c):

    "sphere",   "a=6370997.0",  "b=6370997.0", "Normal Sphere (r=6370997)",

We are running code on a spherical Earth, with a radius (slightly) different from that above.  For consistency sake, proj.4 will need to use the same Earth radius as the code that calls it.  How do I set up a projection on a spherical Earth with a custom radius?

Also... if someone wanted to use proj.4 for mapping on Mars, how would they do that?

Thanks,
-- Bob