embracing operator (`rlang::{{`) not working with default argument

Not entirely sure about your use case, but we can use match.call() to capture the raw call, so cl$arg2 is NULL (not a quosure object), and then using rlang::%||% to select the argument with a value.

library(rlang)
fx  Mazda RX4 Wag     21.0
#> Datsun 710        22.8
#> Hornet 4 Drive    21.4
#> Hornet Sportabout 18.7
#> Valiant           18.1
fx(arg2 = mpg)
#>                    mpg
#> Mazda RX4         21.0
#> Mazda RX4 Wag     21.0
#> Datsun 710        22.8
#> Hornet 4 Drive    21.4
#> Hornet Sportabout 18.7
#> Valiant           18.1

^{Created on 2026-02-13 with reprex v2.1.1}