Role of lHash field in RSAES-OAEP
I believe $\mathit{lHash} \mathbin\Vert \mathit{PS} \mathbin\Vert \mathrm{01}$ serves the rôle of the zero-padding in $s = (m \mathbin\Vert 0^{k_1}) \oplus G(r)$ from the formulation of RSAES-OAEP in the standard reference for its security reduction, on p. 5.  Verifying that it matches exactly what you intend is critical for the security reduction to work, in particular where it figures into Game 6 on p. 14, to provide the bounds involving $k_1$ in Theorem 1 on p. 9.

Using a label perhaps provides some kind of domain separation for different applications, in case, e.g., two application's entropy sources are both broken.