I've run out of time on this, without a successful result.
The best candidate I found for the distro, was Puppy Linux Lupu 511, which is linked to the Ubuntu Lucid repositories, so is easily updated and runs a fairly recent kernel (2.6.34 I think off the top of my head).
It is light enough to install easily on a slow old pentium desktop and has quite a good default interface.
I may well use it in the future if I have to press an old machine into service.
Wasted some time with Lubuntu and a couple of other distros, only to find that they though 4.1GB disk space was 'light'!
I was able to compile a variety of new kernels for Lupu, but hit problems compiling rtai.
The existing PL kernels are quite heavily patched themselves for squashfs, aufs etc and this seemed to conflict with the rtai patch. Rtai patched OK apparently, but then either produced a non-running kernel or one which magma would not compile against.
With more time would have had to sit down and run diff against the rtai .config and the PL .config and try to figure out where the conflicts were and if they were resolvable.
Hope someone else has more success
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