I would think that the Black Carapace would be one of the easier modifications to make to post-convert as it's even given to marines through surgery.
Technically speaking, though, ALL of the gene-seed organs are introduced through surgery. I don't know what the success rate of one organ is versus that of another, but it's all gene-seed at the end of the day. It's not as if it'd be easier to introduce the Black Carapace than the Betcher's Gland to someone who couldn't be an Astartes due to age.
So I would expect that Luther and Kor Phaeron would have worn power armour (or terminator armour) that was fully and properly integrated.
I'm not sure it would be to the same degree, though. One of the things mentioned in 'Descent of Angels' was that the knights of the Order who had been too old to take the gene-seed had been made superhuman... but could never be on par with Space Marines.
The lack of the Black Carapace would be exactly one of the things that would set them apart, I think. I don't believe it's a coincidence, for example, that 'Luther' wore what was described in 'Fallen Angels' as "closer-fitting", less-bulky armour.
The reason this isn't done for Inquisitors and Sisters is likely because either the suits they wear don't have the option for the interface anyway (since they'd be a different model designed for regular humans) or because the Astartes aren't willing to share the process by which the Carapace is implanted.
Agreed on both counts. I can't see how the Adeptus Astartes would suffer gene-seed to be used for individuals other than Space Marines. There is no mention of another process by which an organ like the Black Carapace can be made. Point of fact, I imagine any attempt to duplicate the Emperor's works to such a degree would be seen as blasphemous.
What I find interesting, though, is that the Adeptus Mechanicus haven't figured out a mechanical equivalent. Practically every Techpriest Engineseer wears power armour, but - in game terms at least - it affords them no advantage other than superior protection. With access to bionics (to make them stronger), and proven cybernetic interfaces (see servo-arms), I'm amazed that the Mechanicus hasn't crossed that bridge.
is that it's not worth their effort either on a micro- or a macro-scale. When it comes to leaders and elites, I imagine extensive bionic augmentation provides the same benefits as Astartes power armour (great protection, enhanced strength, etc.). When it comes to their soldiers, the Adepts simply don't care about them enough to make the investment. Skitarii and Praetorians are more than capable versus most threats out there, and the true arm of combat among the Mechanicus has always been the Titan.
Still, I'm curious as to why individual Engineseers wouldn't experiment with cybernetic interfaces for their power armour as an interim solution (since they obviously don't yet possess enough bionics to not need said power armour). You would think it's the sort of initiative they would be rewarded for. Unless augmentation is already a controlled process to begin with, and is used as a reward for a job well done.
Ah, well. I'm rambling.