The princess pony symbols are an interesting thing.
They are made of Electroplate Plastic, that's why they're so shiny. It has an actual property of metal. This can only be done in a factory and it only works on hard plastic items. Things known for it were original She Ra Entrapta doll and the line called Spectra Dolls had it as their entire gimmick. It really is very pretty! It's too bad it can't be done in the home.
I can say that silverleaf DOES work on them, I've done it.
You stick it down with modpodge and then layer modpodge over it. However, leafing is usually a little bit 'crackly' or textured, I've never had it come out glossy/smooth like the originals. I think that you can also enamel paint (aka nail polish the pony no no) on the symbol because it is a completely different plastic than pony-plastic and thus doesn't get sticky or corrupted later like it does when it gets on pony skin. I have seen people do it because enamel is higher spec than metallic acryllics so of course it'd be tempting to use it to fix. But, metal/modpodge is proven to not react with anything so your silver leafing idea is probably best.
*If you go through with it, I'd see before/afters here!