As I understand it, if the magazine was no longer removable the rifle would not be covered by the '94 law.  You could then put all of the preban goodies on that weapon.  I have a spare lower that I have never found a use for, and I was thinking that one of those steel UK mags would be durable and work well.

Anyone know if the BATF has given its blessing to some method?

The issue appears to be "show or go".  If the range and firing are your things, then I have not found an issue with even the 10 round mags. If looks are the issue and limited firing, then permanently mounting the mag should be acceptable, however have found that acceptance is in the eye of the beholder. In other words, modifications that skim the law upon review are just that reviewed and all though you might beat it in court you will still be in court. That could cost far more than a new lower receiver.

It depends on the reviewing official and quite frankly your attitude as you explain your interpetation of the law. I'm trying not to be negative. I own a number of rifles with bayonets. Although I can't see a need, I do like the looks even if it's only sometimes and really don't care for being told you can't. I like flash supressors for the same reason. I really don't shoot at night. Brakes on the other hand though astetic also make firing quickly more effective. If that is you thing, the permenant mounting of the mag is a hindrence since it limits the amount of ammo you fire.

Bottomline is that the law is stupid, but hey what can you do except join the NRA and write letters. "HINT"

I have found over the years that laws are for the law biding and applaud your concern for meeting the letter of the law.