Just do:
data = Dict{String,Any}()
model = Model(HiGHS.Optimizer)
data["x"] = @variable(model, 0 <= x <= 1)
unregister(model, :x)
From a code-readability standard, I think that writing something slightly more verbose that uses the standard JuMP features than trying to writing something custom that saves a line of typing but which is non-standard.