What I'll do is fill in two wishlists, one on the vlan package, the other on ifenslave.Hmmm. How about filing wishlist bug report to ifupdown and provide your script as a patch which falls into one of the example in /usr/share/doc/examples/ . Or create a patch to ifupdown itself to support bondX.Y notation in interfaces and file bug report. This may be enen better.
Please see the thread "Needing explanations about BTS usage for wishlist". Thanks a lot for taking time to answer me :-)