You have almost certainly already tried this because you're an experienced guy, but if not it's worth a shot with plastic miniatures - put poly cement on both pieces, leave it ten seconds or so, then when putting the parts back together, swizzle them about a bit so the melted plastic of one half mingles with the melted plastic of the other half. It goes against the modellers instinct to purposefully melt stuff, but sometimes it's worth it.
Get it positioned properly, support if necessary, then leave it alone for several hours so it's definitely cured before you move it.
You will have a blob of melted gloop to trim off at the joint, but the joint will be welded pretty strongly.
I've done this on things as thin as a Bretonnian knight's lance or a skeleton's wrist and it's been ok.
(Plastic miniatures with long, thin parts are crap aren't they? But I guess a snapped plastic sword is easier to fix than a snapped metal one.)