The Stan Reference Manual has sections: "Affinely Transformed Scalar" - https://mc-stan.org/docs/reference-manual/transforms.html#affinely-transformed-scalar and https://mc-stan.org/docs/reference-manual/types.html#affine-transform.section

The following program compiles:

parameters {
  real<multiplier=3> y;
  vector<multiplier=exp(y/2)>[9] x;
}
model {
  y ~ std_normal();
  x ~ std_normal();
}

here x is a vector, not a scalar. scatter plots of x[1] vs x[2] etc look pretty normal...