Wagner, bom dia!
Supondo que o vetor de valores possíveis é da classe 'expression':
### <code r>
link = "log"
variance = "mu"
quasi(link = link, variance = as.expression(variance))
#
# Family: quasi
# Link function: log
variance = "mu(1-mu)"
quasi(link = link, variance = as.expression(variance))
#
#Family: quasi
#Link function: log
### </code>