You can't boost customs the amount of credits you get in minute compared to the amount needed for you to reach the next rank. Also, there are no commendations for custom games so no bonus credits. It would be quicker to lose all of your games in order to get completion credits and rank up that way than to boost CG...
The above post before mine was made before I posted this so lemme rephrase: The amount of time it would take to get th required credits would be insane, not to mention dull, if ya have the patience for it then sure, but I can't see many people having it...
[Edited on 02.26.2012 1:41 PM PST]