Hey guys.
Are there any NWN scripting verterans on GoG forums who could advise how to change player's portrait and second name on runtime, via a trigger? Is it even possible?

Not me.

But I think you are more likely to get an answer on the scripting forum at the neverwintervault:

https://neverwintervault.org/forums/neverwinter-nights-1/nwn1-scripting

Not possible without nwnx.

There is nwnx plugin that is called names that just changes usernames which are server side
orginally so to hide usernames till you introduced yourself. It hasn't been worked on in while from what I know and can cause crashs (still very buggy)

Portrait can be changed by editing character file in vault but than you need to boot/reload.
How some Race modules change races ect...
http://www.nwnx.org/phpBB2/viewtopic.php?t=1523&postdays=0&postorder=asc&start=2

To do it while player still in game that would need to be almost functionality of both nwncx and nwn and doesn't exist as far as i know.