Not exactly what you are looking for, but you could save PDF X-1a, the RGB images should get converted then.
I consider working with RGB-images in Indesign and separating on exporting the PDFs a viable option (of course the Color Settings should be set and images prepared accordingly).
@Ben – it's always better to do another preflight in AcrobatPro.
So you can be sure nothing slipped through…
And: do not rely on preflight for all problems. Check your pages in Output Preview in AcrobatPro page by page, toggle [Black] separation on/off, "Total Area Coverage", "Color Warnings" …
Check your pages in Output Preview in AcrobatPro page by page, toggle [Black] separation on/off, "Total Area Coverage",
Sound advice. (Well, as long as the length of the document is manageable.)
If you export as PDF/X-1a there's no need to check. The PDF/X-1a standard doesn't allow RGB objects