Couldn't you use a Unique Enchantment to place elemental damage on it? There is already an example in the Digital Grimoire of a sword that could bond with anyone and was dual natured. Also i heard that in previous editions (back when bug city was still big) that you could enchant a weapon with slay bug or some such thing. It wouldn't be that far of a stretch to bind a spell to the weapon and only the person bound to it would not be effected by it.
I have played every edition of Shadowrun and read Bug City cover to cover multiple times. You heard wrong.
The example in the Digital Grimoire has several problems. First, it breaks the fundamental laws of magic. Second, it subjects any mundane wearing it to focus addiction automatically. Third it's fourth Edition. Fourth, as a mundane you REALLY do not want a dual natured weapon.
Fifth, and yeah, this is opinion, but the rules in that book flat out state that the unique enchantment "rules" are there to solve problems that cannot be solved with the rules. Then they basically proceeds to tell you to "make it up as you go". In other words, if game balance or any other part of the lovingly crafted magical theory that is Shadowrun does not satisfy your itch, we have this 18 page cash-in product for you.
Back in my day, this is what we refered to as "Buy The Book" content. (A common house rule in my youth stated that you could only play what you owned. So if you wanted to play with a certain edge or flaw, you had to own the Companion book so the GM could always reference the material. Some books in some systems (*cough* AD&D *cough*) was plain overpowered. Therefore, content that was plain better than everything else was called "Buy The Book" content.)
I am sorry if I come across as condescending or rude. I recognize your intention to help the OP out with a possible solution, but that particular book is a can or worms that best stays closed.