Just in case a simplification makes the question/answer clearer, I'll retry my question from above:

I define a user label:

def p_u_label (expr P,R,S,A)=
    lab:=thelabel(ATTR_text, P)
    process_label(P, R)
enddef;
initsymbol("p_u_label");

This for example will not print a Č character, but the standard Therion label will.

How do I make this display text the same way that a normal Therion label does?

I’ve read through a lot of the Therion source trying to find out where this happens but in the relevant bit there seems to be no extra magic happening.