I need a character to say a sentence to the player including a value. This could be quite a wide range of values so having a condition for each sentence is out of the question. I'm assuming this is not possible with the dialogue system as it would be impossible to have any voice acting for this.

The player is not in conversation at this point, it's said by a character in the background of a scene. Is there any way to have some sort of forced fake subtitles on the screen?

I know I could do it with notification or messagebox but I feel this really kills the immersion, which is the whole point of this mod.

Pretty sure text replacement doesn't work in dialog. You can force dialog for lines, but to my knowledge text replace doesn't work for dialog.

Is the value real, verifiable, and important down to to each single one?

Could it be solved with something like

"We've got 26 comming in, that's horrible" with condition of variable under 30
"We've got 46 on the way, are you sure there's no more?" if it's over 30, under 60
"Ok, 72, we're good till next week" between 60-90

etc etc...
You probably already had an idea like this, but you've gotta go with what you have to work with.