Was in "don't think, do" mental state. In that case

>>BTW
>>

return eval ( 
                       iif (SEEK( tcValue, tcAlias, tcTag )  ;
                          , tcReturnField;
                          , "CAST('  ' as " + type(tcReturnField) +")" ;
                       )

is much shorter and as a one-liner most of the time creates faster running p-code.
The else-branch is now also evaluated, but base expression does not need the icase checking branches depending on parameter type.

As this is now a one-liner, a program could be written to expand the previous udf call in all source files.
@Mike: guessing at the wrapper call frequency and possible optimizations: if your main trouble lies in SQL calling the UDF millions of times, how about thinking to create a &Makroexpression? During loop execution they are a big no-no, but if used in SQL they are Makro-executed only once and the generated source expression is used without penalty for all records, eliminating the wrpper udf call.