As I put my upper together this weekend, one of the handguards that had a crack started to worsen. The tooth is still attached but is starting to flop a bit and I'd like for it to not fall off.

Is there a suggested method to prevent the damage from worsening? My first thought was to carefully apply a little bit of glue to bond it back, but I don't know if that is acceptable for this material or, more importantly, for this kind of original piece. Thankfully this tooth is on the bottom so it won't be very visible regardless, but it is the front most tooth so I bet it does see a bit more strain when installing/removing.

Super glue to the rescue!

+1 When it's dry you can sand off any clumps or excess residue. Careful not to overdo it and melt the tooth!

That was my initial thought. A small amount of CA glue with a careful wipe and then sand the area with some modeling sand paper I have to blend it back in.

I would use a 2 part epoxy and apply it to the inside of the handguard so it is not visible to the outside of the HG.  Some epoxy is black too.  Super glue has never held anything permanently for me.  I fixed the tail light screw attachment tab with epoxy on my suburban the night before driving to Ca.  Worked perfect. Just one of many repairs with epoxy.

I too would use epoxy.  Super glue has low shearing strength,  It holds fine if you try to pull parts apart, but push it sideways and the bond will break.

If I recall, Epoxy dries very hard and brownish.... It's a PITA to cleanup and make look decent without painting over it.

Very small drop of Testors black enamel paint while mixing the epoxy.  A little paint goes a long way.

I have repaired at least six sets that were missing teeth. I used two part epoxy with a little black in it. Candle black, dye and paint all have worked well. I use masking tape as a cast. Dermal tool to shape, sand and polish when done. In your case, you are lucky. The tooth is still there. If it were me, I would mix up a little epoxy with candle black, bend the tooth down, use a tooth pick and dab some in the crack. Clamp or tape it in place and wipe off the excess.

This may sound a bit obvious, but I am guessing "candle black" is black dye used for making candles?

Candle Black, also known as Lamp Black is essentailly carbon soot from candles, lamps and/or lighters.  You can buy it in bulk.  Brownell's, however, sells a specific black epoxy dye for Acra Glas that works in two part epoxy also.