I've only recently got into reading the post-nemesis trek lit, after re-reading New Frontier at the start of the year. Trying to read them in order - halfway through A Singular Destiny at the moment, so I haven't met all of the characters mentioned in this thread...
There are a few mental images i've had reading the books that I've not seen mentioned, so here they are...
Mackenzie Calhoun - I had been picturing Joe Flannigan, like many others, but the voice was never right. On re-read, I've started picturing him as Gabriel Macht.
Soleta - Illustrations vary, but the one that always stuck in my head was the cover of the Quiet Place. I was recently struck by the similarity between that image and Hayley Atwell. Now I picture Soleta as her.
Si Cwan - I could never get a proper image of Cwan, until I saw the original "cast" image (from the covers of books 5 & 6) again - and now I'm seeing him as Titus Welliver. I can't be the only one who thinks there's a resemblance there, right?
Gleau - not seen him mentioned anywhere. I visualized him as Craig Parker.
Nan Bacco - probably because I read Articles of the Federation directly after watching some Borgen, I initially visualized her as Sidse Babett Knudsen. After a while I started seeing her as Cherry Jones. Now I kinda switch between the two.
Christine Vale - I started out picturing her as Katee Sackhoff, but ever since watching In To Darkness, I've been picturing her as Alice Eve.
T'Ryssa Chen - The first image that came to mind when i read about the character was Lian T'Su from The Arsenal Of Freedom (Julia Nickson). And the image kinda stuck.