Total Guess on my part - but may be worth an experiment.
Modelers who built and modified Historex Plastic Figures made a mixture of extra plastic sprue or parts in tenax or other thin plastic weld. They would drop the plastic in a partially filled jar of the liquid and let it melt, adding plastic or more weld to get the proper consistency. The stuff was applied with a brush, a separate bottle of weld was used to keep the brush clean and provide further thinning as required.
Once the seam is filled - fine sanding - maybe even the toothpaste route would restore a smooth seam and surface.
Unfortunately the seam may still have a different glow in the dark look to it, since the basic plastic structure has been changed.