Is there any way to access your user journal entries after you've finished the game? I've just been writing rather detailed ones for once and I wouldn't mind a copy, but I don't know how.

Yeah just reload your last save and check your journal enteries. They should still be there.

If you mean how to take a copy you better learn the ways of the famus PrtScn button. (just hit that button ingame if you have the ScrnShot folder in the BG2 folder)

if u save them in a .char file i think you kan edit/copy the user journals via. shadowkeeper or the cvhar viewer