I have written up a feature narrative about making admin buildpacks available to meta-buildpacks like the heroku-buildpack-multi, which delegates staging to other buildpacks. The narrative also includes a link to some exploratory code that shows how this feature could be used by the multi-buildpack:

https://docs.google.com/document/d/1APcx1EbdRDtGiUfXEbiM2-7Sq-wxMGS85a5ICt65HTc/edit#

Please leave any comments inside the document. This kind of feedback is an important indicator to the CF Foundation to show if there is community interest in a particular feature, or not!

Cheers,
-Jan