I spent a few minutes looking around and experimenting and I did not see a way to do it. I don't know everything you're trying to do, however while it may not be your preference this technique might work for you:

:SETVAR DB_NAME_ASSIGN "TEST TEXT"

:SETVAR DB_NAME_TEXT "EXPECT INSERT OF DB_NAME_ASSIGN HERE: "

/*Get output*/

SELECT '$(DB_NAME_TEXT)' + '$(DB_NAME_ASSIGN)' AS OUTPUT

/* Delivers:

EXPECT INSERT OF DB_NAME_ASSIGN HERE: TEST TEXT

*/