Answering my own question...
It didn't strike me first that Image-XChange SDK is actually included along with PDF-XChange Pro SDK since versions 5.x.
That is *great* (old) news.
Do you want to know why I missed it? Its documentation is missing from the PDF-XChange Pro SDK download. I had to download the Image-XChange SDK kit to get to it.
Now to my task at hand: it actually revolves around the following, thanks to the Image-XChange SDK:
- browse the images using PXCp_ImageGetFromPage()
- get them as Image-XChange objects through PXCp_GetDocImageAsXCPage()
- check their encoding format using IMG_PageGetFormat()
- if needed, convert them to my needed grayscale 8 bpp format using IMG_PageConvertToFormat()
- get access to the array of bytes representing the pixels through IMG_PageLockBlock()
- do my stuff from there
- then cleanup resources, which involves IMG_PageUnlockBlock(), IMG_PageDestroy(), PXCp_ImageClearPageData(), and later PXCp_ImageClearAllData()
It works *really* well.