Plugging material must, by rule, be similar to but not necessarily identical to the ball material. Yes, metals are out. There is also a rule that states you cannot increase the weight of a bowling ball to greater than the manufactured weight, which is only practically relevant if you wish to plug/slug very light balls.
I explored this when the 'no balance hole rule' was proposed, and a noted coach was expressing concern about the restrictions it would impose on bowlers. My response was - drill the balance hole, 3/4 fill with polystyrene foam and plug over the top.
There are no rules AFAIK regarding the density of plug, but realistically there is not much of a range available. Not too many people have access to stuff like barium salts to bulk up the density, and even fewer have the technical skills to successfully tweak rg, diff and bias, so I'd class that as a non-issue unless the USBC changes the rules dramatically. If they do, my drilling prices will go up.
My issue with vinyl slugs is that they are very soft, decreasing the integrity of the slug (not a problem if it's deep enough, I guess) and that they leak plasticiser and dye like a sieve.
\soapbox...
Super glue should not be used for slugs except for emergencies. It is both harder than the ball and brittle, so impacts on the slug shatter the glue and the attached coverstock. That initiates cracks which propagate through the coverstock, generating the typical annular cracking seen around many thumb slugs. It is also a moisture curing glue, and often won't cure properly in a freshly drilled hole (super glue relies on the thin layer of water present on the surface of everything in contact with the air to bond) so occasionally the slugs loosen or fall out. 5 minute Araldite takes a bit longer to cure, but gives a permanent, secure fix and virtually eliminates cracking.
/soapbox.
