Eric,
It's just not technically realistic. Code Replacement doesn't know anything about any of the other fields you would want to also have the replacement applied to. It's even hard to thing of what the UI would be like that would allow you to dictate which fields in which dialogs that a given code would apply.
Generally when I see requests like this one (and there have been many) I have to think that there is a different way to accomplish the same thing. And in this case there is. If you put your code (without the delimiters) in some unused IPTC field in the first pass through your images and then use a specially prepared IPTC Stationery Pad, you can achieve the effect you're looking for in bulk.
In your IPTC Stationery Pad, use the {variable} for the unused field, (let's say you use Transmission Reference, then you would use {transref}) in all the places you want your expansion to occur. You can additionally use the extended syntax of Code Replacement to choose alternate replacements that are even more suitable for your use in other fields. It would look like this:
\{transref}\
\{transref}#2\
\{transref}#3\
etc.
When you apply your IPTC Stationery Pad to your entire take, the value you entered into the Transmission Reference field will be extracted and put in place of the {transref} in each of your Code Replacements. This method does take two steps, but minimizes the amount of times you have to enter your delimiter characters.
Let's say you're doing NBA Basketball and you're identifying players by their jersey number and team name. You have codes like this for the LA Lakers and the Boston Celtics:
LA24 Kobe Bryant Kobe Bryant, LA Lakers #24 LA Lakers
LA85
...
BC12
BC46
...
As you go through your take in the IPTC Info dialog you identify players and put the player code in the Transmission Reference field without the delimiters and then Save and Advance to the next photo (e.g. you enter LA23). When you get to the end of your photos you apply your IPTC Stationery Pad that has fields prepared like so:
Caption: \{transref}#2\ makes a field goal at the Lakers/Celtics game on {iptcdate}.
Keywords: \{transref}\, \{transref}#3
etc.
Now, all of the fields will be expanded at once and with richer information than simply expanding the same value into multiple fields. It doesn't take much longer than your feature suggestion would have used (it actually may be faster since you don't have to type delimiters and you can prepare the IPTC Stationery Pad settings in advance of the game.)
HTH,
-Kirk
