Hello:
Actually I don't agree that qualcú is the Balearic equivalent of qualsevol. Here we use qualsevol and qualcú for different meanings.
Qualcú means "anyone", "someone", "someone you don't know", the Spanish equivalent is "alguien", while the Spanish equivalent for "qualsevol" is "cualquiera"
Ex:
Hi ha qualcú?: Is there anyone?
Qualcú m'ha robat les claus: Alguien me ha robado las llaves
Qualsevol m'ha pogut robar les claus: Cualquiera me ha podido robar las llaves.