Apparently my math for the background was correct, but my GL-fu is not strong enough. To be correct, it needs to render with vertex colour clamping disabled (and only clamp at the last minute before writing to the colour buffers), for which there is a convenient extension method glClampColorARB (defined by the ARB_color_buffer_float extension). At least, it would be convenient if Mesa
actually implemented it. Which, apparently, it doesn't. *sigh*
This seems to leave me with the options of a) rendering the backplane in software and uploading it as a texture (which could get complex real first once all options (texturing, etc) are supported), and b) computing all the clamp lines in the viewport and rendering a separate polygon for each clamped segment. Neither of which is particularly elegant. I suppose there's also the question of whether I'm eventually going to (have to?) do a pure software renderer or not.
Ah poo. Apparently I've misunderstood the purpose of the glClampColorARB call. As it turns out, Nvidia
's driver + libraries do include it, but it appears to have no effect in this context.
 Yes, I'm willing to assume that Nvidia engineers probably have a better understanding of the extension "spec" than I do.