First off, this is not to whine about not being able to take a Black Templar Librarian. I have no problems with the fact that that particular chapter doesn't field them.
That said, the Black Templars
do
have recruits with psychic potential, not all of whom are purged as heretical witches. The old White Dwarf article which gave us tabletop rules for Deathwatch kill-teams also mentioned that not many chapters send their Librarians to the Deathwatch, quite understandably viewing them as too valuable a resource.
However, the Deathwatch apparently has their own Librarians, taken and trained from scratch from the recruits of chapters (like the Black Templars) who refuse to make use of them.
The same article also implied, if not outright stated, that the majority of Deathwatch Librarians were home-grown from the psychic rejects of rabidly anti-psyker chapters.
If this is the case in your campaigns (or even if it wasn't, and you simply had someone who wanted to play one anyway), how would you handle it, ruleswise?
(I'm intrigued, as my own instinct is to go with a slightly tweaked Black Shield. If your response would be to disallow it, fair enough, but it's not massively helpful)