Its a cool idea, we have some peeps doing 500 series API stuff for DIY (both commercial and non-commercial) and if you hang on a few months(ish) there should be a new rack system in production that is open standard and will have lots of cool DIY options plus commercial ones and it may even fit API500 with an adapter card......but thats all I should say.
The module size will be quite small though, so fitting PPM14 meters for G-SSL etc can't be done. The smaller the size the harder the DIY.
I think a 6-space (12HP wide - 60.6mm) euro subrack is the best compromise, should be able to squeeze a G-1176 or G-SSL easily....with cool meters.
Tom