The Gaming Element has interviewed ProfessionalVictim from HL-Nightwatch. The interview is available here: http://www.thegamingelement.com/articles.php?articleid=4

Here is a quote for you:
TGE: Now we know Nightwatch will be re-entering the Half-Life world at the same time Gordon and Barney and all that lot are still in the Black Mesa complex. Is there any possibility that we would have say an encounter with one of the original characters from Half-Life?

ProfessionalVictim: As currently planned, there won't be any direct interaction with the player characters from the original Half-Life or its expansions. One of our goals is to produce an entirely new adventure for the player instead of simply building on top of the existing ones and borrowing content from them. It would also require the reproduction of several level areas at a much higher detail level without access to any of the original map. The player will, however, be aware of the actions of the other player characters throughout the episode, and these actions do affect the events in Nightwatch. Some elements in Nightwatch will also tie in more directly with elements from the previous stories.