Can you post a photo of it? I doubt that it's really fixable. As you say, no glue is going to work, even with drilled pins and wires through the joint, it will come apart because the sideframe is under pressure to keep the wheels in. You could try tacking it in position with ACC just to have it in the right spot, and then heat melting across the break with a soldering iron. Then I would get some scrap plastic and melt it into the joint and across it to beef it up.
Depending on where the break is, if you can bridge it with a shim of brass and put screws through there, that would be a very reliable fix. But if it snapped off right between the bolster and the sideframe, there won't be any room for that.
If that doesn't hold...
I'd probably just rebuild the underside of the tender with Kato GS-4 tender trucks. You will still have to do some scratchbuilding to make them fit, but they are close in appearance and are really excellent trucks. They are available on kato's website for $15 a pair.