Plastic trigger guard? Wonder if you could glue that sucker till you can get it replaced? Should function fine, keep it in on safe till your ready to shoot and uncap that sucker getting in and out of your stand.
It might not look real pretty but some o the two part jb weld or plumbers putty ($5 at lowes) will at least re-attach it to potentially aid with the safety function of the trigger guard. I'd then be looking to replace it... Maybe even the whole muzzleloader???