You seem to be mixing two things here:
- Whether we can use hipsgen out of the box to generate HiPS maps from tangent-plane coadds, or whether we need to write our own code in the afw framework to compute HEALPix-gridded fluxes from our tangent-plane coadds.
- If neither of the above works, whether we need to re-coadd from scratch in the HEALPix pixel space in order to get accurate results.
I am in no way claiming confidence that hipsgen will do the proper resampling, to the level of quality that is our general goal in our image processing algorithms. I'm making a different point, which is that if it can't be done at all without re-coadding, then there seems to be something deeper wrong in our ability to understand the coadded flux and to do things like provide a no-fuss cutout service. If we can't produce reliable flux estimates because our tangent-plane images are "highly distorted" at the edges, that applies to all uses of them, not just conversion to HEALPix, and we should be fixing that, as it would imply, for example, that the generic coadd cutout service would also need to redo coadds from scratch just to make properly tangential cutouts centered in the overlap regions.
My mental model of all this has long been that, assuming that our tangent-plane coadds and associated algorithms are good enough, with the edge distortions understood well enough, we would in fact write our own code to produce the base layer of the HiPS hierarchy, i.e., an all-sky image, in HEALPix coordinates, computed using afw from our tangent coadds. We could then still use hipsgen to roll it up into a full HiPS tree.
Maybe this is what you meant all along, but I interpreted you to be saying that the coaddition itself would need to be re-done in HEALPix space.