How to set secondary framebuffer resolution via bootparam?

Hello!

How can I change the default resolution from 640x400 to 640x480 of the sm501
secondary (panel) channel for kernel _boot_ time?

The sm501 driver creates two framebuffer drivers (in-kernel 2.6.24 driver):
/dev/fb0 is sm501-crt
/dev/fb1 is sm501-panel

What I've tried:
console=/dev/ttyS0,115200 fbcon=map:1 video=sm501fb:640x480

The fb starts successfully on /dev/fb1, the penguin-logo appears
but the resolution cannot be changed to the proper 640x480 needed
for my panel.

The syntax isn't also clear to me. I don't see any help in the
Documentation. I want to change the video mode of the second
framebuffer. How can I do that?
I've tried several versions:

vga=...
sm501-fb:...
sm501fb:mode:640x480...
sm501-panelfb:640x480

What is the syntax here, to get the _secondary_ fb set poperly.

Any dirty source code hack like s/400/480/g is also welcome, it's quite
urgent but only for a simple demonstration. Kernel .config is attached.
That's a sm501 framebuffer driver working on an embedded powerpc's PCI bus
(shouldn't matter).

Setting the resolution via fbset would be possible, but that's
not the point here!

Thank you,

Clemens