I'd just go with leaving them frozen in level until their XP allows them to catch up to their LA. It's not as if your campaign will stay forever, or even very long, at first level, since, frankly, 1st level characters suck and are utterly worthless, not to mention way too easily to accidentally kill when you didn't intend to even try to do so.
DMing first level characters is one of the least satisfying things to do, personally, because you spend most of your time pulling your punches on characters which, frankly, haven't had any room whatsoever to develop.
On the flip side, as a player, I don't really invest any real attachment to first-level characters either, since I fully expect them to die and thus consider them entirely expendable. After all, it'll take me all of a minute to roll up another one just like it, and point-buy further reduces the value of a character at all, since when all characters are the same, you're simply churning out mass-produced stamped-from-sheet-metal characters. Perfect time to be pulling a Desert Gorge.