Conditional in <phrase>

[ringleader]

I'm trying to replace the username across the forum with a Userfield of my own.

However, I'm running into difficulty with the double quotes (I think).

I'm trying to replace

$bbuserinfo[username]

with

<if condition="$bbuserinfo[fieldX]">$bbuserinfo[fieldX]<else />$bbuserinfo[username]</if>

However, many templates use $bbuserinfo[username] in phrases like:

<phrase 1="$bbuserinfo[username]"></phrase>

Desired result:

<phrase 1="$bbuserinfo[username]"></phrase>

when $bbuserinfo[fieldX] exists, but

<phrase 1="$bbuserinfo[fieldX]"></phrase>

when $bbuserinfo[fieldX] exists.

How would I go about replacing my conditional in there, or is it possible at all?